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D R Weaver

Publications and source records attributed to D R Weaver.

At least 73 records · Page 4Linked to original sources
PubMed1991

Melatonin receptors and signal transduction during development in Siberian hamsters (Phodopus sungorus).

Maternal melatonin communicates daylength information to the fetus in Siberian hamsters. Fetal sensitivity to melatonin declines near birth. In this report, we describe melatonin receptor distribution and a second messenger response to melatonin in Siberian hamsters during the perinatal period. The sites of high-affinity 2-[125I]iodomelatonin ([125I]MEL) binding were generally similar throughout the perinatal period. The non-hydrolyzable GTP analog, guanosine-5'-O-(3-thiotriphosphate) (100 microM) inhibited [125I]MEL binding at each age, suggesting the melatonin receptors are associated with guanine nucleotide binding proteins (G proteins). Furthermore, melatonin (10 nM) inhibited forskolin-stimulated cAMP accumulation in median eminence/pars tuberalis (ME/PT) explants as early as 4 days before birth, when sensitivity to melatonin in vivo is high. The cAMP regulatory system appeared disrupted on the day of birth, in that forskolin (10 microM) stimulation of cAMP accumulation was reduced, and melatonin did not inhibit cAMP accumulation stimulated by forskolin. A higher forskolin dose (100 microM) elevated cAMP levels more clearly on the day of birth, and melatonin inhibited forskolin-stimulated cAMP accumulation. These results suggest that the decreased physiological responsiveness to melatonin at the end of gestation may be due to alterations in the cAMP regulatory system.

Animals↗
PubMed1991

Melatonin receptors and signal transduction in photorefractory Siberian hamsters (Phodopus sungorus).

In seasonally breeding mammals, seasonal alterations in day length are perceived via the pineal hormone melatonin. When exposed to short day lengths, hamsters and other long-day breeders undergo gonadal regression. With chronic exposure (greater than 20 weeks) to short days, however, the animals become photorefractory, or insensitive to the inhibitory influence of short-day melatonin patterns, and gonadal recrudescence occurs. In this report, we examined photorefractory Siberian hamsters and long-day housed control hamsters to examine whether this apparent insensitivity to melatonin is due to alterations in melatonin receptors or signal transduction. In vitro autoradiographic assessment of melatonin receptors using 125I-labeled 2-iodomelatonin (I-MEL) revealed that melatonin receptor distribution, affinity, density, and G protein coupling are unaltered in photorefractory animals. In each animal, high-affinity (dissociation constant approximately 40 pM) 2-iodomelatonin binding sites were observed in the hypophysial pars tuberalis, in the suprachiasmatic nucleus of the hypothalamus, and in the thalamus (paraventricular nucleus, reuniens nucleus, and nucleus of the stria medullaris). The nonhydrolyzable GTP analog, GTP gamma S (100 microM) caused a 10-fold reduction in melatonin receptor affinity in the pars tuberalis in both photorefractory and control hamsters, demonstrating receptor-G protein coupling in both groups. Furthermore, melatonin (10 nM) inhibited forskolin-stimulated cAMP accumulation in median eminence/pars tuberalis explants in photorefractory animals, just as previously observed in explants from long-day hamsters. These results suggest that melatonin receptors, receptor-G protein coupling, and inhibition of adenylyl cyclase by melatonin are not altered in photorefractory hamsters.

Animals↗
PubMed1991

Molecular cloning and characterization of a rat A1-adenosine receptor that is widely expressed in brain and spinal cord.

An A1-adenosine receptor has been cloned from a rat brain cDNA library using a probe generated by the polymerase chain reaction. The cDNA encodes a protein of 327 amino acids which is 91% identical to a recently cloned dog A1-adenosine receptor (RDC7). Expression of the rat cDNA in COS-6M and NIH 3T3 cells resulted in ligand binding and functional activity characteristics of an A1-adenosine receptor that is coupled to inhibition of adenylyl cyclase. Examination of the distribution of A1-adenosine receptor mRNA by Northern blot analysis showed that it is highly expressed in brain, spinal cord, testis, and white adipose tissue. In situ hybridization studies revealed an extensive hybridization pattern in the central nervous system, with high levels in cerebral cortex, hippocampus, cerebellum, thalamus, brainstem, and spinal cord. The cloned A1-adenosine receptor may thus mediate many of the modulatory actions of adenosine in neural and endocrine systems.

Amino Acid Sequence↗
PubMed1990

Melatonin receptors and signal transduction in melatonin-sensitive and melatonin-insensitive populations of white-footed mice (Peromyscus leucopus).

The pineal hormone melatonin times seasonal alterations in reproductive function in photoperiodic mammals. In white-footed mice, there is variation in responsiveness to the reproductive effects of melatonin between populations originating in different locations; mice from Connecticut (CT) respond normally to melatonin, while mice from Georgia (GA) appear insensitive to melatonin. In the present paper, we compare melatonin receptor distribution and a second messenger response to melatonin in white-footed mice from CT and GA. Specific binding of 125I-labeled melatonin (I-MEL) was observed in a variety of brain regions in each population, but there were no consistent differences in the distribution or intensity of I-MEL binding between the populations. Furthermore, melatonin inhibited forskolin-stimulated cAMP accumulation in median eminence/pars tuberalis explants from both populations. These results suggest that insensitivity to melatonin in GA mice is not due to a gross defect in melatonin receptors or receptor-effector coupling.

Animals↗
PubMed1990

The distribution of melatonin binding sites in neuroendocrine tissues of the ewe.

Specific high-affinity binding of 2-[125I]iodomelatonin (IMEL) was examined in 20-micrometer sections prepared from intact Suffolk ewes killed during late anestrus or the breeding season. The pars tuberalis contained by far the highest concentration of IMEL binding sites of all areas studied. Within the telencephalon, intense labeling was found in the mediolateral septum, the ventrolateral septal and septohypothalamic nuclei, the entorhinal cortex, the subiculum, and the inner and outer molecular layers of the hippocampus adjacent to the dentate gyrus. Melatonin binding in the medial preoptic nucleus, bed nucleus of the stria terminalis, and medial preoptic area was less striking but still distinct. Among diencephalic regions, melatonin binding sites existed in low concentrations in the anterior hypothalamus, the tuberal medial basal hypothalamus, and the paraventricular thalamic and supramammillary nuclei. Little binding was evident in the suprachiasmatic or ventromedial nuclei. In the midbrain, significant binding was restricted to the ventral raphe complex and the inferior colliculus. Little specific binding was found in the pars distalis or the pineal gland. The distribution of melatonin binding in the sheep brain is discussed in the context of the influence of this pineal hormone upon seasonal changes in neuroendocrine function.

Animals↗
PubMed1990

Melatonin receptors are present in the ferret pars tuberalis and pars distalis, but not in brain.

The pineal hormone melatonin regulates reproductive function in seasonally breeding mammals. Recent studies using 125I-labeled 2-iodomelatonin (I-MEL) reveal that the distribution of putative melatonin receptors is species-specific; only the hypophysial pars tuberalis (PT) is a consistent site of I-MEL binding in all photoperiodic species examined. In the present study, we used in vitro autoradiography to examine the distribution of I-MEL binding in the ferret brain and pituitary. We report that I-MEL binding is restricted to the PT and pars distalis (PD) of the pituitary; I-MEL binding is absent from brain. I-MEL binds in the PT and PD with high affinity (Kd values ca. 40 pM) and the rank order of potency for inhibition of I-MEL binding (6-chloromelatonin = melatonin greater than 6-hydroxymelatonin greater than N-acetylserotonin greater than serotonin) is the same as that observed for high-affinity melatonin receptors from other species. The consistent presence of high affinity melatonin receptors in the PT of a variety of photoperiodic species suggests that the PT plays a major role in mediating the effects of melatonin on neuroendocrine function.

Animals↗
PubMed1989

Direct in utero perception of light by the mammalian fetus.

The primary neural pathway for entrainment of circadian rhythms in rodents is the retinohypothalamic tract, which conveys lighting information from the retina to a biological clock in the hypothalamic suprachiasmatic nuclei (SCN). In a precocious rodent species, the spiny mouse, the retinohypothalamic tract is present and functioning within the SCN on the day of birth, as assessed by HRP histochemistry and 14C-labeled 2-deoxyglucose autoradiography, respectively. Furthermore, direct perception of environmental lighting was observed in fetal spiny mice late in gestation. Direct, retina-mediated fetal light perception appears to be a less potent entraining agent than maternal cues communicating time-of-day information to the fetal spiny mouse. Nevertheless, direct fetal light perception may reinforce maternal entraining signals during the prenatal period and therefore be of physiological significance for entrainment of circadian rhythmicity in the fetus.

Animals↗
PubMed1989

Periodic feeding of SCN-lesioned pregnant rats entrains the fetal biological clock.

Destruction of the maternal suprachiasmatic nuclei (SCN) early in gestation disrupts maternal communication of time-of-day information to the rat fetus. In the present study, we demonstrated that periodic feeding (food cue) to SCN-lesioned pregnant rats entrains the fetal biological clock. The phase of the drinking behavior rhythm was examined in pups reared in constant darkness, beginning at weaning. In several control (uncued) litters, pup phases at weaning were scattered. In other control litters where within-litter coordination of phase was observed, the average litter phase was unpredictable. In contrast, drinking rhythms of pups whose dams had received food cue during gestation were synchronized within- and between-litters, suggesting that prenatal food cue entrained the fetuses. The effect of food cue occurs prenatally, as similar results were obtained when offspring of SCN-lesioned, cued dams were fostered to lesioned, uncued dams on the day of birth. The present results, along with data from this and other laboratories, suggest that redundant mechanisms communicate time-of-day information to the fetus.

Animals↗
PubMed1989

Melatonin receptors in chick brain: characterization and localization.

Melatonin receptors in chick brain were characterized by RRA and localized by in vitro autoradiography, using [125I]melatonin [( 125I]MEL), a biologically active melatonin analog. In membranes from whole brain, radioreceptor studies revealed a high affinity [125I]MEL binding site with an equilibrium dissociation constant of 47.2 +/- 11.5 (mean +/- SEM) pM and a density of 37.8 +/- 8.5 fmol/mg protein. Binding was reversible and competitively inhibited by melatonin and closely related melatonin analogs, but not be norepinephrine or serotonin. In vitro autoradiographic studies of brain revealed a widespread distribution of melatonin receptors. Specific I-MEL binding was observed in retinorecipient and integrative nuclei of the visual system including the avian homolog of the mammalian suprachiasmatic nuclei. Auditory relay nuclei and limbic structures associated with arousal and vocalization were also specifically labeled. This widespread distribution of putative melatonin receptors in chick brain is in marked contrast to the very restricted distribution of melatonin receptors in mammalian brain and suggests that avian sensory systems are affected by melatonin.

Animals↗
PubMed1989

Melatonin signal transduction in hamster brain: inhibition of adenylyl cyclase by a pertussis toxin-sensitive G protein.

Melatonin signal transduction was examined in median eminence/pars tuberalis (ME/PT) explants from Djungarian hamsters. High affinity melatonin receptors in hamster ME/PT were first quantified by in vitro autoradiography using the potent melatonin agonist 125I-labeled melatonin ([125I]MEL). Scatchard analysis of [125I]MEL binding in ME/PT revealed high affinity receptors [dissociation constant (Kd) = 2.75 X 10(-11) M]. [125I]MEL binding was markedly reduced by guanine nucleotides; treatment with the nonhydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) caused a 10-fold decrease in receptor affinity. Melatonin (10 nM) significantly inhibited forskolin-stimulated cAMP accumulation in ME/PT, but not in pituitary or pineal glands. In ME/PT explants, melatonin and 6-chloromelatonin inhibited forskolin-stimulated cAMP accumulation in a dose-dependent manner with similar potency (significant inhibition for each at concentrations greater than or equal to 100 pM). Serotonin significantly inhibited forskolin-stimulated cAMP levels only at doses greater than or equal to 100 microM. Inhibition of [125I]MEL binding in ME/PT by these three indolamines paralleled that determined for inhibition of forskolin-stimulated cAMP accumulation. Pertussis toxin treatment (1 microgram/ml) blocked the ability of melatonin (10 nM) to inhibit forskolin-stimulated cAMP accumulation and significantly reduced [125I]MEL binding. Pertussis toxin ADP-ribosylated the alpha-subunits of at least two guanine nucleotide-binding proteins in ME/PT explants with molecular weights of approximately 40 K. Melatonin did not increase phosphodiesterase activity in ME/PT explants. The results strongly suggest that a signal transduction pathway for melatonin in mammals involves inhibition of adenylyl cyclase by a pertussis toxin-sensitive guanine nucleotide-binding protein.

Adenylate Cyclase Toxin↗
PubMed1989

Localization and characterization of melatonin receptors in rodent brain by in vitro autoradiography.

Little is known of the neural sites of action for the pineal hormone, melatonin. Thus, we developed an in vitro autoradiographic method using 125I-labeled melatonin (I-MEL) to study putative melatonin receptors in rodent brain. We first determined optimal in vitro labeling conditions for autoradiographic detection of I-MEL binding sites in rat median eminence, the most intensely labeled area in the rat brain. We then assessed the pharmacologic and kinetic properties of I-MEL binding sites in rat median eminence by quantitative autoradiography. These sites have high affinity for I-MEL (equilibrium dissociation constant = 43 pM). I-MEL binding was inhibited by nanomolar concentrations of melatonin or 6-chloromelatonin, but was not inhibited by serotonin, dopamine, or norepinephrine (100 microM). These results suggest that I-MEL binding sites identified by in vitro autoradiography represent specific, high-affinity melatonin receptors. Studies of the distribution of I-MEL binding in rat, Syrian hamster, and Djungarian hamster brain confirm that the median eminence and suprachiasmatic nucleus are major sites of I-MEL binding in rodent brain; other brain areas labeled in one or more of these species were the thalamus (paraventricular, anteroventral, and reuniens nuclei, nucleus of the stria medullaris, and medial part of the lateral habenular nucleus), hypothalamus (dorsomedial nucleus), subiculum, and area postrema. The presence of putative melatonin receptors in the suprachiasmatic nuclei and median eminence of these rodent species suggests that these brain regions are important loci for melatonin effects on circadian rhythms and reproduction.

Animals↗
PubMed1988

Putative melatonin receptors in a human biological clock.

In vitro autoradiography with 125I-labeled melatonin was used to examine melatonin binding sites in human hypothalamus. Specific 125I-labeled melatonin binding was localized to the suprachiasmatic nuclei, the site of a putative biological clock, and was not apparent in other hypothalamic regions. Specific 125I-labeled melatonin binding was consistently found in the suprachiasmatic nuclei of hypothalami from adults and fetuses. Densitometric analysis of competition experiments with varying concentrations of melatonin showed monophasic competition curves, with comparable half-maximal inhibition values for the suprachiasmatic nuclei of adults (150 picomolar) and fetuses (110 picomolar). Micromolar concentrations of the melatonin agonist 6-chloromelatonin completely inhibited specific 125I-labeled melatonin binding, whereas the same concentrations of serotonin and norepinephrine caused only a partial reduction in specific binding. The results suggest that putative melatonin receptors are located in a human biological clock.

Autoradiography↗
PubMed1988

Iodinated melatonin mimics melatonin action and reveals discrete binding sites in fetal brain.

Iodinated melatonin was used to study melatonin sites of action in brain. Iodomelatonin mimicked the effects of melatonin on reproductive development in Djungarian hamster fetuses. 125I-melatonin injected into the dam was recovered from fetal brain. In vitro autoradiographic studies revealed a remarkably discrete distribution of competitive 125I-melatonin-binding sites in the fetal brain, with binding in median eminence/arcuate nucleus area greater than suprachiasmatic nucleus greater than pineal gland much greater than anterior pituitary gland much greater than preoptic area. 125I-melatonin promises to be a useful tool for understanding the sites and mechanism of action of melatonin.

Animals↗
PubMed1988

Retention of masculine sexual behavior following castration in male B6D2F1 mice.

The reduction of masculine sexual behavior following castration varies widely among genotypes. In contrast to the loss of sexual behavior by castrated males of other strains, males of the B6D2F1 genotype retain the ejaculatory reflex for many weeks after castration. The present study examined this retention phenomenon. Masculine sexual behaviors were measured before and after castration or sham operation in male C57BL/6J, DBA/2J, and B6D2F1 mice. Castrated C57BL/6J and DBA/2J males showed a rapid decline in copulatory behavior. In contrast, 30% of the B6D2F1 males continued to ejaculate 25 weeks after castration. Regardless of whether or not sexual behaviors were retained, levels of plasma testosterone and hypothalamic nuclear estrogen receptors were reduced by castration. These results suggest that the intra- and inter-strain differences in the retention of sexual behavior following castration are not due to differences in levels of steroid hormones. Further, some B6D2F1 males retain the ability to copulate in the absence of gonadal hormone levels required for the maintenance of sexual behavior in other genotypes.

Animals↗
PubMed1988

Hormonal restoration of masculine sexual behavior in long-term castrated B6D2F1 mice.

In contrast to the facilitative effects reported for other rodents, testosterone treatment at the time of castration previously was reported to inhibit masculine sexual behavior in male B6D2F1 mice. Males of this genotype vary in their behavioral response to castration. Some castrates retain sexual behaviors for many weeks after surgery, whereas others do not. In the present study, we sought to determine the effects of exogenous steroid hormone treatment on castrated B6D2F1 mice that had ceased to show copulatory behavior. Testosterone propionate and estradiol benzoate restored copulatory behavior to precastration levels in B6D2F1 males that did not retain sexual behaviors after castration.

Animals↗
PubMed1988

Maternal communication of circadian phase to the developing mammal.

In rodents, an entrainable circadian clock begins oscillating prenatally in the hypothalamic suprachiasmatic nuclei (SCN). The maternal circadian system coordinates (entrains) the timing of the developing clock to the prevailing light-dark cycle during both late fetal and early neonatal life. This maternal communication of circadian phase ensures that the developing animal is coordinated to the outside world until maturation of the normal pathway of light-dark entrainment in adults, the retinohypothalamic pathway, permits direct photic entrainment through the neonatal eye. The mechanism of maternal communication of circadian phase remains to be determined, but the necessity of the maternal SCN in this communication has been demonstrated. Indirect lines of evidence suggest that a similar scheme occurs for the development of circadian rhythms in humans.

Animal Communication↗
PubMed1988

Djungarian hamsters exhibit reproductive responses to changes in daylength at extreme photoperiods.

In seasonally breeding species, an animal's photoperiodic history (the daylength or photoperiod previously experienced) influences the reproductive response to new photoperiods. However, this has only been examined over a relatively narrow range of photoperiods. We assessed whether Djungarian hamsters (Phodopus sungorus) respond to daylength changes at extremely long and extremely short photoperiods. To determine the extent and temporal sequence of reproductive responses to photoperiod changes, ultrasonography was employed to determine serial changes in testis weight in individual animals; serial changes in body weight and pelage color were also assessed. Male animals (60-day-old) shifted from 6 h of light/day (6L) to 10L showed partial testicular recrudescence and darkening of the coat. In control animals remaining in 6L, testes remained small, and coat color continued to lighten. Animals shifted from 20L to 16L showed partial and transient testicular regression, whereas testes remained large in control animals remaining in 20L. There were no significant differences in body weight between control and experimental animals in either study. These findings indicate that Djungarian hamsters respond reproductively to changes in photoperiod at extreme daylengths, but the magnitude of the response appears to be dependent on the absolute daylength.

Animals↗