Effect of MT1 melatonin receptor deletion on melatonin-mediated phase shift of circadian rhythms in the C57BL/6 mouse.

In the mouse suprachiasmatic nucleus (SCN), melatonin activates MT1 and MT2 G-protein coupled receptors, which are involved primarily in inhibition of neuronal firing and phase shift of circadian rhythms. This study investigated the ability of melatonin to phase shift circadian rhythms in wild type (WT) and MT1 melatonin receptor knockout (KO) C57BL/6 mice. In WT mice, melatonin (90 microg/mouse, s.c.) administered at circadian time 10 (CT10; CT12 onset of activity) significantly phase advanced the onset of the circadian activity rhythm (0.60 +/- 0.09 hr, n = 41) when compared with vehicle treated controls (-0.02 +/- 0.07 hr, n = 28) (P < 0.001). In contrast, C57 MT1KO mice treated with melatonin did not phase shift circadian activity rhythms (-0.10 +/- 0.12 hr, n = 42) when compared with vehicle treated mice (-0.12 +/- 0.07 hr, n = 43). Similarly, in the C57 MT1KO mouse melatonin did not accelerate re-entrainment to a new dark onset after an abrupt advance of the dark cycle. In contrast, melatonin (3 and 10 pm) significantly phase advanced circadian rhythm of neuronal firing in SCN brain slices independent of genotype with an identical maximal shift at 10 pm (C57 WT: 3.61 +/- 0.38 hr, n = 3; C57 MT(1)KO: 3.45 +/- 0.11 hr, n = 4). Taken together, these results suggest that melatonin-mediated phase advances of circadian rhythms of neuronal firing in the SCN in vitro may involve activation of the MT2 receptor while in vivo activation of the MT1 and possibly the MT2 receptor may be necessary for the expression of melatonin-mediated phase shifts of overt circadian activity rhythms.

The antidepressant-like effect of the melatonin receptor ligand luzindole in mice during forced swimming requires expression of MT2 but not MT1 melatonin receptors.

We previously reported an antidepressant-like effect in C3H/HeN mice during the forced swimming test (FST) following treatment with the MT1/MT2 melatonin receptor ligand, luzindole. This study investigated the role melatonin receptors (MT1 and/or MT2) may play in the effect of luzindole in the FST using C3H/HeN mice with a genetic deletion of either MT1 (MT1KO) or MT2 (MT2KO) melatonin receptors. In the light phase (ZT 9-11), luzindole (30 mg/kg, i.p.) significantly decreased immobility during swimming in both wild type (WT) (135.6 +/- 25.3 s, n = 7) and MT(1)KO (132.6 +/- 13.3 s, n = 8) as compared with vehicle-treated mice (WT: 207.1 +/- 6.0 s, n = 7; MT1KO: 209.5 +/- 6.2 s, n = 8) (P < 0.001). In the dark phase (ZT 20-22), luzindole also decreased time of immobility in both WT (89.5 +/- 13.9 s, n = 8) and MT1KO (66.5 +/- 6.4 s, n = 8) mice as compared with the vehicle treated (WT: 193.8 +/- 3.5, n = 6; MT1KO: 176.6 +/- 6.2 s, n = 8) (P < 0.001). Genetic disruption of the MT1 gene did not alter the diurnal rhythm of serum melatonin in MT1KO mice (ZT 9-11: 1.3 +/- 0.6 pg/mL, n = 7; ZT 20-22: 10.3 +/- 1.1 pg/mL, n = 8) as compared with WT (ZT 9-11: 1.4 +/- 0.7 pg/mL; ZT 20-22: 10.6 pg/mL). Swimming did not alter the serum melatonin diurnal rhythm in WT and MT1KO mice. Decreases in immobility of WT and MT1KO mice by luzindole treatment were not affected by gender or age (3 months versus 8 months). In contrast, luzindole did not decrease immobility during the FST in MT2KO mice. We conclude that the antidepressant-like effect of luzindole may be mediated through blockade of MT2 rather than MT1 melatonin receptors.

Short-term exposure to melatonin differentially affects the functional sensitivity and trafficking of the hMT1 and hMT2 melatonin receptors.

The hormone melatonin mediates a variety of physiological functions in mammals through activation of pharmacologically distinct MT(1) and MT(2) G protein-coupled melatonin receptors. We therefore sought to investigate how the receptors were regulated in response to short melatonin exposure. Using 2-[(125)I]iodomelatonin binding, cAMP functional assays, and confocal microscopy, we demonstrated robust differences in specific 2-[(125)I]iodomelatonin binding, receptor desensitization, and cellular trafficking of hMT(1) and hMT(2) melatonin receptors expressed in Chinese hamster ovary (CHO) cells after short (10-min) exposure to melatonin. Exposure to melatonin decreased specific 2-[(125)I]iodomelatonin binding to CHO-MT(2) cells (70.3 +/- 7.6%, n = 3) compared with vehicle controls. The robust decreases in specific binding to the hMT(2) melatonin receptors correlated both with the observed functional desensitization of melatonin to inhibit forskolin-stimulated cAMP formation in CHO-MT(2) cells pretreated with 10 nM melatonin (EC(50) of 159.8 +/- 17.8 nM, n = 3, p < 0.05) versus vehicle (EC(50) of 6.0 +/- 1.2 nM, n = 3), and with the arrestin-dependent internalization of the receptor. In contrast, short exposure of CHO-MT(1) cells to melatonin induced a small decrease in specific 2-[(125)I]iodomelatonin binding (34.2 +/- 13.0%, n = 5) without either desensitization or receptor internalization. We conclude that differential regulation of the hMT(1) and hMT(2) melatonin receptors by the hormone melatonin could underlie temporally regulated signal transduction events mediated by the hormone in vivo.

Melatonin receptor signaling: finding the path through the dark.

Melatonin, dubbed "the hormone of darkness," is involved in relaying photoperiodic information to the organism. Not only is melatonin involved in the regulation of circadian rhythms and sleep, but it also has roles in visual, cerebrovascular, reproductive, neuroendocrine, and neuroimmunological functions. Melatonin mediates its effects through G protein-coupled receptors: MT(1), MT(2), and, possibly, MT(3). Pharmacological agents have been instrumental in identifying these receptor types. Masana and Dubocovich discuss how the level of receptor expression may alter their efficacy, so that caution is necessary when extrapolating the pharmacological properties of ligands defined on recombinant systems to the receptors in the organism. With these cautions in mind, they describe the various signaling pathways and physiological roles ascribed to the three melatonin receptor types.

Circadian rhythm of mt1 melatonin receptor expression in the suprachiasmatic nucleus of the C3H/HeN mouse.

This report studied the diurnal and circadian rhythms of mt1 melatonin receptor expression in the SCN of C3H/HeN mice maintained in either a light:dark (LD) cycle or in constant dark for a minimum of 6 wk. Diurnal times (ZT) were assessed with reference to the onset of the light period (ZT0) and circadian times (CT) were established by determining the phase of wheel running activity of each mouse before sacrifice. 2-[125I]-Iodomelatonin binding in the SCN revealed low amplitude diurnal and circadian rhythms with highest levels of binding 2 hr after lights on (41.3+/-1.7 fmol/mg protein, n = 5, at ZT2) or at the beginning of the subjective day (48.6+/-2.1 fmol/mg protein, n = 6, CT2), respectively. The expression of mt1 mRNA, determined by in situ hybridization with a 35S-labeled mouse mt1 riboprobe, showed robust diurnal and circadian rhythms. In animals housed under a LD cycle, low levels of expression were observed during the day, with a rapid rise in mt1 melatonin receptor expression at the beginning of the dark period (ZT14), coincident with an abrupt increase in levels of circulating melatonin measured by radioimmunoassay. In animals housed under constant dark conditions, a robust peak of mt1 mRNA expression occurred in the middle of the subjective night (CT18), 8 hr before the peak of protein expression, while the lowest levels of mt1 mRNA expression were observed during the day (CTI10). Results suggest that mt1 melatonin receptor rhythm in the C3H/HeN mouse SCN is regulated both by light and by the biological clock as distinct rhythms of both mRNA and protein are differentially expressed under a LD cycle and constant dark conditions.

Lithium regulates PKC-mediated intracellular cross-talk and gene expression in the CNS in vivo.

It has become increasingly appreciated that the long-term treatment of complex neuropsychiatric disorders like bipolar disorder (BD) involves the strategic regulation of signaling pathways and gene expression in critical neuronal circuits. Accumulating evidence from our laboratories and others has identified the family of protein kinase C (PKC) isozymes as a shared target in the brain for the long-term action of both lithium and valproate (VPA) in the treatment of BD. In rats chronically treated with lithium at therapeutic levels, there is a reduction in the levels of frontal cortical and hippocampal membrane-associated PKC alpha and PKC epsilon. Using in vivO microdialysis, we have investigated the effects of chronic lithium on the intracellular cross-talk between PKC and the cyclic AMP (cAMP) generating system in vivo. We have found that activation of PKC produces an increase in dialysate cAMP levels in both prefrontal cortex and hippocampus, effects which are attenuated by chronic lithium administration. Lithium also regulates the activity of another major signaling pathway the c-Jun N-terminal kinase pathway--in a PKC-dependent manner. Both Li and VPA, at therapeutically relevant concentrations, increase the DNA binding of activator protein 1 (AP-1) family of transcription factors in cultured cells in vitro, and in rat brain ex vivo. Furthermore, both agents increase the expression of an AP-1 driven reporter gene, as well as the expression of several endogenous genes known to be regulated by AP-1. Together, these results suggest that the PKC signaling pathway and PKC-mediated gene expression may be important mediators of lithium's long-term therapeutic effects in a disorder as complex as BD.

Interactions between light and melatonin on the circadian clock of mice.

Melatonin and light synchronize the biological clock and are used to treat sleep/wake disturbances in humans. However, the two treatments affect circadian rhythms differently when they are combined than when they are administered individually. To elucidate the nature of the interaction between melatonin and light, the present study assessed the effect of melatonin on circadian timing and immediate-early gene expression in the suprachiasmatic nucleus (SCN) when administered in the presence of light. Male C3H/HeN mice, housed in constant dark in cages equipped with running wheels, were treated with either melatonin (90 microg, s.c.) or vehicle (3% ethanol-saline) 5 min prior to exposure to light (15 min, 300 lux) at various times in the circadian cycle. Combined treatment resulted in lower magnitude phase delays of circadian activity rhythms than those obtained with light alone during the early subjective night and advances in phase when melatonin and light were administered during the subjective day (p < .001). The reduction in phase delays with combined treatment at Circadian Time (CT) 14 was significant when light exposure measured 300 lux but not at lower light levels (p < .05). When light preceded melatonin administration, the inhibition of phase delays attained significance only when the light exposure reached 1000 lux (p < .05). Neither basal nor light-induced expression of c-fos mRNA in the SCN was modified by melatonin administration at CT 14 or CT 22. Together, these results suggest that combined administration of melatonin and light affect circadian timing in a manner not predicted by summing the two treatments given individually. Furthermore, the interaction is not likely to be due to inhibition of photic input to the clock by melatonin but might arise from a photically induced enhancement of melatonin's actions on circadian timing.

Selective MT2 melatonin receptor antagonists block melatonin-mediated phase advances of circadian rhythms.

This study demonstrates the involvement of the MT2 (Mel1b) melatonin receptor in mediating phase advances of circadian activity rhythms by melatonin. In situ hybridization histochemistry with digoxigenin-labeled oligonucleotide probes revealed for the first time the expression of mt1 and MT2 melatonin receptor mRNA within the suprachiasmatic nucleus of the C3H/HeN mouse. Melatonin (0.9 to 30 microg/mouse, s.c.) administration during 3 days at the end of the subjective day (CT 10) to C3H/HeN mice kept in constant dark phase advanced circadian rhythms of wheel running activity in a dose-dependent manner [EC50=0.72 microg/mouse; 0.98+/-0.08 h (n=15) maximal advance at 9 microg/mouse]. Neither the selective MT2 melatonin receptor antagonists 4P-ADOT and 4P-PDOT (90 microg/mouse, s.c.) nor luzindole (300 microg/mouse, s.c.), which shows 25-fold higher affinity for the MT2 than the mt1 subtype, affected the phase of circadian activity rhythms when given alone at CT 10. All three antagonists, however, shifted to the right the dose-response curve to melatonin, as they significantly reduced the phase shifting effects of 0.9 and 3 microg melatonin. This is the first study to demonstrate that melatonin phase advances circadian rhythms by activation of a membrane-bound melatonin receptor and strongly suggests that this effect is mediated through the MT2 melatonin receptor subtype within the circadian timing system. We conclude that the MT2 melatonin receptor subtype is a novel therapeutic target for the development of subtype-selective analogs for the treatment of circadian sleep and mood-related disorders.

Physiological exposure to melatonin supersensitizes the cyclic adenosine 3',5'-monophosphate-dependent signal transduction cascade in Chinese hamster ovary cells expressing the human mt1 melatonin receptor.

Here, we report the effects of short exposure to melatonin on the human mt1 (h mt1) melatonin receptor-mediated signaling in Chinese hamster ovary (CHO) cells, and the consequences of an exposure that resembles the physiological pattern of melatonin release on cAMP-mediated signal transduction. Short exposure (10 min) of h mt1 melatonin receptors to melatonin (400 pM) inhibited forskolin-stimulated cAMP formation, cAMP-dependent protein kinase activity, and phosphorylation of the cAMP response element-binding protein. However, treatment of mt1-CHO cells with melatonin in a manner that closely mimics the in vivo activation of melatonin receptors (i.e. 400 pM melatonin for 8 h to mimic darkness) resulted in a supersensitization of the cAMP-dependent signal transduction cascade during the period of withdrawal (i.e. 16 h without melatonin to mimic the light cycle of a diurnal photoperiod). During the period of withdrawal, forskolin induced a time-dependent (1-16 h) increase in cAMP formation (approximately 200% of control cells). This effect of melatonin was dependent on the presence of the h mt1 melatonin receptor, as no potentiation of forskolin-induced cAMP formation was observed in CHO cells transfected only with the neomycin resistance plasmid. The time-dependent increase in forskolin-stimulated cAMP levels resulted in a potentiation of cAMP-dependent protein kinase activity 1 h after withdrawal (approximately 130% of control cells; P < 0.05) and in the number of cells containing the phosphorylated form of cAMP response element-binding protein (approximately 75% of cells at 1 and 16 h compared with 30% in control cells; P < 0.05). An increase in the undissociated state (G alphabetagamma) of Gi proteins may underlie this phenomenon as demonstrated by the increase in pertussis toxin-catalyzed ADP-ribosylation of G proteins (217 +/- 48% of control; P < 0.05) after melatonin withdrawal. This increase in the ribosylation was not due to an up-regulation of Galpha(i) protein, as no significant change in Galpha(i) protein levels occurred at this time. We demonstrated that activation of the h mt1 melatonin receptor in a manner that resembles the physiological pattern of melatonin exposure alters signaling, as potentiation of cAMP-mediated signal transduction events is observed after hormone withdrawal. The CHO cells expressing the human melatonin receptor may provide an in vitro cellular model in which to investigate the putative signaling mechanisms leading to gene regulation by melatonin.

Melatonin receptor antagonists that differentiate between the human Mel1a and Mel1b recombinant subtypes are used to assess the pharmacological profile of the rabbit retina ML1 presynaptic heteroreceptor.

We have identified subtype selective agonists, partial agonists and antagonists, which distinguish the human recombinant Mel1a and Mel1b melatonin receptors expressed in COS-7 cells. Melatonin receptor agonists showed higher affinity for competition of 2-[125I]-iodomelatonin binding for the Mel1b than the Mel1a melatonin receptor. The dissociation constants (Ki) of 16 agonists determined on the recombinant human Mel1a and Mel1b melatonin receptor subtypes showed a significant correlation (r2 = 0.85, slope = 0.97, P < 0.0001, n = 16). However, six agonists showed 10 to 60 fold higher affinity for the Mel1b melatonin receptor as indicated by the affinity selectivity ratios (Mel1a/Mel1b) [8-methoxy-2-acetamidotetraline (11); S20098 (14); 8-methoxy-2-propionamidotetraline (20); 6, 7 di-chloro-2-methylmelatonin (21); 6-chloromelatonin (57); 6-methoxymelatonin (59)]. Dissociation constants for competition of 11 partial agonists and antagonist for 2-[125I]-iodomelatonin binding were between 15.5 (luzindole, pKi: 7.7) to 362 (4-phenyl-2-chloroacetamidotetraline, pKi: 9.1) fold higher for the Mel1b than for the Mel1a melatonin receptor. The lack of correlation between the pKi values (r2 = 0.23, P > 0.1, n = 11) strongly suggest that the two human melatonin receptor subtypes can be distinguished pharmacologically. The partial agonist: 5-methoxyluzindole (pKi: 9.6) and the competitive melatonin receptor antagonists: GR128107 (pKi: 9.6), 4-phenyl-2-chloroacetamidotetraline (pKi: 9.1), 4-phenyl-2-acetamidotetraline (pKi: 8.9) and 4-phenyl-2-propionamidotetraline (pKi: 8.8) are selective Mel1b melatonin receptor analogues as their affinity selectivity ratios (Mel1a/Mel1b) are bigger than 100. We conclude that the 40% overall amino acid difference in the sequence of the human recombinant Mel1a and Mel1b melatonin receptors is reflected in distinct pharmacological profiles for the subtypes. We compared the pharmacological profile of the presynaptic ML1 melatonin heteroreceptor of rabbit retina mediating inhibition of the calcium-dependent release of dopamine to that of the recombinant Mel1a and Mel1b melatonin receptors. Melatonin inhibited [3H]dopamine release by 50% (1C50) at 20 pM with a maximal inhibitory effect (80%) at 1 nM. The partial agonists, i.e., N-acetyltryptamine (1C50 5.6, maximal inhibition 55%) and 5-methoxyluzindole (1C50: 1.3, maximal inhibition 40%) showed various degrees of efficacy while none of the competitive melatonin receptor antagonists did inhibit [3H]dopamine release on their own. The potency (1C50) of full melatonin receptor agonists significantly correlated with their affinity to compete for 2-[125I]-iodomelatonin binding to either the Mel1a (r2 = 0.76, slope = 0.77, P < 0.0001, n = 17) or Mel1b (r2 = 0.63, slope = 0.75, P < 0.001, n = 17) human melatonin receptors. By contrast, the apparent dissociation constants (KB) for partial agonists and antagonists to antagonize the inhibition of [3H]dopamine release mediated by activation of the ML1 heteroreceptor by melatonin, significantly correlated with the affinity constants (Ki) for 2-[125I]-iodomelatonin binding determined of the Mel1b (r2 = 0.77, slope = 0.55, P < 0.001; n = 11) but not the Mel1a (r2 = 0.27, P < 0.1, n = 11) subtype. Together these results demonstrate that the pharmacological profile of the human recombinant Mel1b melatonin receptor is similar to that of the functional presynaptic melatonin heteroreceptor of rabbit retina, which we referred as an ML1B subtype. We conclude that the selective Mel1b melatonin partial agonists and antagonists described here can be used to identify melatonin receptor subtypes in native tissues and to search for subtype selective analogues with therapeutic potential.

Light-induced phase shifts of circadian activity rhythms and immediate early gene expression in the suprachiasmatic nucleus are attenuated in old C3H/HeN mice.

Alterations in the mechanisms of entrainment and/or response of the circadian pacemaker to zeitgebers may contribute to age related changes in sleep/wake rhythms. This study examined the effect of age on light-induced phase shifts of circadian activity rhythms and on the expression of the immediate early genes c-fos and jun-B in the suprachiasmatic nucleus (SCN) of young and old C3H/HeN mice. Mice (4 months or 16 months at the beginning of the experiment) were housed in constant darkness with circadian rhythms assessed by running wheel activity. Mice were exposed to light pulses of 30, 100, 300 or 1000 lux and steady state phase shifts of circadian activity rhythms determined. In young mice exposed to light at circadian time (CT) 14, light pulses of 30, 100, 300 or 1000 lux induced phase delays of circadian activity rhythms of similar magnitude (averaging 2.8 h). Phase delays following photic stimulation were reduced in the old mice at all light levels (averaging 1.1 h, P < 0.001). Following behavioral testing, mice were exposed to light (1000 lux) at CT 14 for determination of the light-induced expression of c-fos and jun-B mRNA in the SCN by in situ hybridization histochemistry. Immediate early gene expression following light exposure was reduced by 42% (c-fos) and 48% (jun-B) in the SCN of old mice compared to young controls (P < 0.001). Together, these results suggest an age related reduction in responsiveness to light by the circadian pacemaker.

Responsiveness to melatonin and its receptor expression in the aging circadian clock of mice.

This study determined the effect of age on the efficacy of melatonin treatment to phase shift circadian activity rhythms and on melatonin receptor expression in the suprachiasmatic nucleus (SCN) and paraventricular nucleus of the thalamus (PVNT) of C3H/HeN mice. The circadian rhythm of 2-[125I]iodomelatonin binding, assessed at three times of the day [circadian times (CT) 2, 10, and 18], showed a modest age-related decrease in the SCN but not the PVNT of old C3H/HeN mice (24 mo). There was a tendency for age to reduce Mel1a melatonin receptor mRNA expression in the suprachiasmatic nucleus during the day, but not during the night. The magnitude of phase shifts of circadian activity rhythms (advances or delays) induced by administration of melatonin at CT 10 or CT 2 was identical in young and old C3H/HeN mice. Together, these results suggest that the decrease in melatonin receptor expression in the SCN had little effect on melatonin-induced phase shifts of circadian activity rhythms. We conclude that the responsiveness of the circadian timing system to melatonin administration does not decrease with age.

Light-induced c-fos mRNA expression in the suprachiasmatic nucleus and the retina of C3H/HeN mice.

Light-induced expression of c-fos mRNA was studied over a circadian period (approximately 24 h) in C3H/HeN mice maintained in constant dark. This mouse strain expresses an rd mutation (retinal degeneration) which does not affect light-induced phase shifts of circadian rhythms. c-fos mRNA expression in the retina and the suprachiasmatic nucleus (SCN) after a light pulse (300 lux) was determined by in-situ hybridization autoradiography using a 35S-labeled c-fos riboprobe. Light induced the expression of c-fos mRNA in retino-recipient areas of the SCN. This response was dependent on the circadian time (CT) and was observed only during the subjective night (CT14-CT22) and early subjective day (CT2). However, the period of photosensitivity for c-fos induction extended 1 h over the period of photosensitivity for phase shifts in circadian behavior. In the retina of C3H/HeN mice, light-induced c-fos mRNA expression was observed in a small number of cells in the ganglion cell layer (approximately 0.2%) which may represent ganglion cells projecting to the SCN. A dependence of c-fos expression with the circadian time was observed in retinal ganglion cells, suggesting that retinal photosensitivity may also be controlled by a circadian oscillator. In conclusion, we demonstrated light-induced expression of the immediate early gene c-fos mRNA in both the retina and SCN of C3H/HeN mice expressing the rd mutation.

Regulation of signal transduction pathways by mood-stabilizing agents: implications for the delayed onset of therapeutic efficacy.

A series of investigations were performed to elucidate the mechanisms of action of lithium, valproate, and carbamazepine. We have found that lithium exerts major effects on G proteins, most likely via a posttranslational process stabilizing the inactive heterotrimeric (alpha beta gamma) form of the protein. We also find that chronic lithium and valproate exert major, very similar effects on the PKC signaling pathway, with both drugs decreasing the levels of membrane-associated PKC alpha and epsilon, and have similar effects on the DNA binding activity of the transcription factor, AP-1. By contrast, we find that carbamazepine exerts major, direct inhibitory effect at the level of adenylyl cyclases. Overall, the results suggest that signal transduction pathways are targets for the actions of mood-stabilizing agents; given their key roles in the amplification and integration of signals in the central nervous system, these findings have clear implications not only for research into the etiology/pathophysiology of manic-depressive illness, but also for the development of innovative treatment strategies.

Melatonin receptors in the mammalian suprachiasmatic nucleus.

The SCN of the hypothalamus, the site of the circadian pacemaker in mammals, is endowed with melatonin receptors of the ML-1 subtype. Here, we present evidence suggesting that activation of melatonin receptors in the SCN regulates circadian rhythms of behavior in the mouse. In a paradigm simulating a eastbound transmeridian flight, timed administration of melatonin may either accelerate or decrease the rate of reentrainment. Moreover, under constant environmental conditions, exogenous melatonin phase shifts circadian rhythms only during times when the production of the hormone is inhibited. Similarly, light shows periods of circadian sensitivity only at times when light is not present in a natural photoperiod. The maximal phase shifts elicited by melatonin and light coincide with the subjective light-dark (dusk) and subjective dark-light (dawn) transitions. The periods of sensitivity for melatonin, occur at the same circadian times in mouse and in man. Under a short photoperiod the duration of the nocturnal melatonin production may overlap with periods of sensitivity for the hormone, and therefore, melatonin, may be important in synchronizing circadian rhythms to changes in the natural photoperiod. It follows that the identification of periods of circadian sensitivity to melatonin in mammals is important for the development of effective treatments with melatonin and related analogues for sleep disorders characterized by alterations of circadian rhythmicity.

Cloning and characterization of a new member of the G-protein coupled receptor EDG family.

We report here the cloning of a new member of the endothelium differentiation gene (edg) subfamily of G-protein-coupled receptors. This novel cDNA sequence was cloned from the ovine pars tuberalis using a reverse transcriptase polymerase chain reaction (RT-PCR) amplification with degenerate primers homologous to the highly conserved II and VII transmembrane domains of the G-protein coupled receptor gene family. The PCR product was random primed with 32P and used as a probe to screen a size-selected cDNA ovine pars tuberalis library, which resulted in the isolation of a single clone of 2700 bp. This novel sequence was named edg-2, because its nucleic acid sequence was 55% homologous over 501nt overlap to an orphan sequence cloned from human endothelial cells, the endothelial differentiation gene, edg-1. The highest degree of aminoacid homology (42%) occurs in the seven putative transmembrane domains, particularly between the transmembrane domains III and VI (53% and 64%, respectively). The intervening hydrophilic domains are short and there are numerous putative phosphorylation sites for Ser/Thr-protein kinases in the second and third intracellular loop and in the COOH-terminal domain. Through Northern analysis of total RNA, low levels of at least four transcripts of 2.3, 2.5, 3.2 and 4 kb were found in sheep cerebral cortex and a 4.2 kb transcript was observed in NIH/3T3 fibroblasts. In addition, edg-2 transcripts (415 bp) were amplified by RT-PCR from pars tuberalis, cerebral blood vessels, hypothalamus, and retina. Serum stimulation of Chinese hamster ovary (CHO) cells expressing the edg-2 receptor resulted in increased cell proliferation, as measured by [3H]-thymidine incorporation. Edg-1 and edg-2 appear to be distinct genes that may encode protein products that bind the same or related ligand.

In vivo evidence that nonneuronal beta-adrenoceptors as well as dopamine receptors contribute to cyclic AMP efflux in rat striatum.

We applied in vivo microdialysis to assess the effects of dopaminergic and beta-adrenergic receptor stimulation on cyclic AMP efflux in rat striatum under chloral hydrate anesthesia. Dopamine (up to 1 mM) infused for 20 min through the probe did not increase cyclic AMP, whereas both the selective dopamine D1 agonist SKF 38393 and D2 antagonist sulpiride produced modest increases. It is interesting that the beta-adrenoceptor agonist isoproterenol produced a marked increase (204.7% of basal level at 1 mM) which was antagonized by the beta-adrenoceptor antagonist propranolol. Pretreatment with a glial selective metabolic inhibitor, fluorocitrate (1 mM), by a 5-h infusion through the probe attenuated basal cyclic AMP efflux by 30.3% and significantly blocked the response to isoproterenol. By contrast, striatal injection of a neurotoxin, kainic acid (2.5 micrograms), 2 days before the dialysis experiment did not affect basal cyclic AMP or the response to isoproterenol, but blocked the response to SKF 38393. These data demonstrate the beta-adrenoceptors as well as dopamine receptors contribute to cyclic AMP efflux in rat striatum in vivo. They also suggest that basal and beta-adrenoceptor-stimulated cyclic AMP efflux are substantially dependent on intact glial cells.

In vivo evidence that lithium inactivates Gi modulation of adenylate cyclase in brain.

In vivo microdialysis of cyclic AMP from prefrontal cortex complemented by ex vivo measures was used to investigate the possibility that lithium produces functional changes in G proteins that could account for its effects on adenylate cyclase activity. Four weeks of lithium administration (serum lithium concentration of 0.85 +/- 0.05 mM; n = 11) significantly increased the basal cyclic AMP content in dialysate from prefrontal cortex of anesthetized rats. Forskolin infused through the probe increased dialysate cyclic AMP, but the magnitude of this increase was unaffected by chronic lithium administration. Inactivation of the inhibitory guanine nucleotide binding protein Gi with pertussis toxin increased dialysate cyclic AMP in control rats, as did stimulation with cholera toxin (which activates the stimulatory guanine nucleotide binding protein Gs). The effect of pertussis toxin was abolished following chronic lithium, whereas the increase in cyclic AMP after cholera toxin was enhanced. In vitro pertussis toxin-catalyzed ADP ribosylation of alpha i (and alpha o) was increased by 20% in prefrontal cortex from lithium-treated rats, but the alpha i and alpha s contents (as determined by immunoblot) as well as the cholera toxin-catalyzed ADP ribosylation of alpha s were unchanged. Taken together, these results suggest that chronic lithium administration may interfere with the dissociation of Gi into its active components and thereby remove a tonic inhibitory influence on adenylate cyclase, with resultant enhanced basal and cholera toxin-stimulated adenylate cyclase activity.

Dopamine synthesis precedes dopamine uptake in embryonic rat mesencephalic neurons.

We have measured [3H]dopamine ([3H]DA) uptake and tyrosine hydroxylase-immunopositive immunostaining in cells acutely dissociated from the embryonic ventral mesencephalon (MSC). DA and its metabolites as well as catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) activities were determined in homogenates taken from the MSC and striatum (STR). In the embryonic ventral MSC measurable DA and tyrosine hydroxylase (TH) immunostaining were present as early as embryonic day (E) 12.5. At E14 the number of TH+ neurons was about 50% of the values at E18. In the MSC, DA concentration increased sharply at E16 and reached a plateau before birth that was 10-fold lower than adult values. In the STR, DA was first detected at E16, suggesting that DA fibers reach the STR at this embryonic stage. High-affinity DA uptake appeared in the MSC only at E16, concomitantly with the arrival of DA fibers in the STR, increased sharply between E16 and E18, and reached a plateau before birth. This uptake mechanism was not selective for catecholamine uptake inhibitors. Thus, DA synthesis in the MSC preceded the onset of high-affinity uptake mechanism, which could be correlated to the beginning of striatal DA innervation. Measurable MAO and COMT activities were detected as early as E13 (MSC) and E15 (STR), but not DA metabolites, which appeared later. We conclude that the high-affinity DA uptake mechanism in MSC DA neurons develops coincident with the arrival of DA fibers to the STR. The sharp increase of DA uptake between E16 and E18 is due only in part to an increase in the number of TH+ cells. These results support the hypothesis that in vivo the target STR neurons regulate the maturation of MSC DA cells.