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Biomedical subjects

M K Vaughan

Publications and source records attributed to M K Vaughan.

At least 91 records · Page 5Linked to original sources
PubMed1984

Modification of testicular and thyroid function by chronic exposure to short photoperiod: a comparison in four rodent species.

Exposure of male Syrian hamsters (Mesocricetus auratus) for 10 weeks to short photoperiod (SP) providing 10 hr light: 14 hr darkness (10:14 LD) produced a significant reduction in the weights of the reproductive organs, plasma thyroxine (T4) levels and free T4 index (FT4I) compared to the values of animals exposed to long photoperiod (LP, 14:10 LD). C57bl male house mice (Mus musculus) kept in SP (10:14 LD) had reproductive organ weights equivalent to those of mice kept in long days (14:10 LD) and lower T3 uptake (T3U) values. Male gerbils (Meriones unguiculatus) exposed to 13 weeks of SP (10:14 LD) had lower body weights, testes and seminal vesicle weights and higher T3U values compared to LP (14:10 LD) controls. However, no effect was seen on plasma T4 and triiodothyronine (T3) values nor the FT4I and free T3 index (FT3I). White-footed male mice (Peromyscus leucopus) exposed to SP (8:16 LD) had significantly lower testes and seminal vesicle weights while plasma T4 and T3 levels were unaffected. Snell strain house mice (Mus musculus) exposed to SP (8:16 LD) had normal reproductive organ weights compared to the values of LP-exposed (16:8 LD) control animals. However, there was a significant depression in T3 and in the FT3I in the SP animals.

Animals↗
PubMed1984

Inhibitory action of the pineal gland on the nuclear volume of thyroid follicular cells in male gerbils (Meriones unguiculatus).

Karyometric investigations of thyroid follicular cells were performed in four groups of adult male gerbils [I. sham-operated (Sham PX) + short photoperiod (SPP)-exposed; II. Sham PX + long photoperiod (LPP)-exposed; III. pinealectomized (PX) + SPP-exposed; IV. PX + LPP-exposed]. The mean nuclear volume (MNV) of thyroid follicular cells was significantly higher in both PX groups (III and IV) when compared to Sham PX controls (I and II). In Sham PX gerbils the maintenance of animals in SPP (10 h light: 14 h darkness) for 13 weeks resulted in significant decrease of MNV of thyroid follicular cells in comparison to that in Sham PX animals kept in LPP (14 h light: 10 h darkness). Lighting conditions did not significantly affect the MNV of thyroid follicular cells in PX gerbils. These data suggest that the pineal gland itself and not light deprivation is the most important factor depressing the activity of the thyroid in male gerbils. Lighting conditions seem to affect the function of thyroid follicular cells inasmuch as they activate or block the action of the pineal gland.

Animals↗
PubMed1984

Relative efficacy of melatonin and 5-methoxytryptamine in terms of their antigonadotrophic and counterantigonadotrophic actions in male Syrian hamsters.

Antigonadotrophic and counterantigonadotrophic activities of melatonin and 5-methoxytryptamine (5-MT) were quantitatively compared in male Syrian hamsters. In long day conditions, the daily afternoon administration of either 5, 15, 25, 50, 100 or 200 micrograms melatonin induced testicular regression within 10 wk; under the same circumstances, only the 200-microgram dosage of 5-MT was able to suppress testicular weights. Thus, 5-MT appears to have about one-tenth the antigonadotrophic action of melatonin in the male Syrian hamster. In short days, the subcutaneous implantation of either 50 or 100 micrograms melatonin (every 2 wk in beeswax) prevented testicular regression whereas it required much larger doses of 5-MT (1 mg every 2 wk in beeswax) to achieve the same counterantigonadotrophic action. In terms of both their antigonadotrophic and counterantigonadotrophic effects, hamsters seem to be more sensitive to melatonin than to 5-MT.

5-Methoxytryptamine↗
PubMed1984

Opioid and dopamine involvement in prolactin release induced by arginine vasotocin and vasopressin in the male rat.

The potential involvement of the endogenous opioid and dopamine (DA) systems in the mechanism(s) mediating arginine vasotocin (AVT)- and arginine vasopressin (AVP)-induced prolactin (PRL) release was investigated in vivo. The injection of AVT (5 micrograms) into unanesthetized male rats resulted in a 2-fold stimulation of PRL release 15 min later, followed by an inhibition of PRL release 30 min thereafter; both the stimulatory and inhibitory PRL responses to AVT were obviated by naloxone (NAL) (200 micrograms). Similarly, the administration of either AVT or AVP (5 micrograms) to urethane-anesthetized rats led to a 3- and 5-fold increase in plasma PRL levels, respectively, 10 min after injection. The PRL stimulatory response to both peptides was completely blocked by pretreating the animals with apomorphine (APO) (5 mg); however, the injection of APO by itself had no effect on PRL secretion in these animals. Both AVT and AVP were also effective in stimulating PRL release 10 min after injection in estrogen (50 micrograms)-progesterone (25 mg) (EP)-treated rats anesthetized with urethane. APO negated the PRL stimulatory response to these compounds in the EP-treated rat as well. Normal, urethane-treated rats experienced a 7- to 8-fold increase in PRL levels 20 min following the injection of methysergide (MET) (250 micrograms). Both AVT and AVP caused approximately a 2.5-fold greater PRL response in MET-treated animals than in AVT and AVP controls, respectively; however, only in the MET + AVT-treated rats was the PRL stimulatory response greater than in the MET controls. MET probably stimulated PRL through its DA antagonistic properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Effects of injections and/or chronic implants of melatonin and 5-methoxytryptamine on plasma thyroid hormones in male and female Syrian hamsters.

Plasma thyroid hormone concentrations in male and female Syrian hamsters were evaluated in five experiments after two pineal indoles, melatonin (MEL) and 5-methoxytryptamine (5-MT), were administered as chronic s.c. implants and/or daily afternoon injections. Circulating concentrations of thyroxine (T4), ordinarily maintained by the long photoperiod (LD 14:10), were inhibited by daily afternoon injections of 25 micrograms MEL but not 5-MT in male (experiment 1) and female (experiment 2) Syrian hamsters. The suppressive effect of MEL injections on T4 concentration in the long photoperiod was prevented in both sexes by a s.c. beeswax pellet containing either 1 mg MEL or 5-MT. In the third experiment, various doses of MEL or 5-MT were injected each afternoon for 12 weeks to compare the effectiveness of these two indoles in reducing T4 concentrations in hamsters maintained in the long photoperiod. Only the highest dose (200 micrograms) of 5-MT effectively suppressed T4 concentrations whereas all doses of MEL (greater than or equal to 5 micrograms) significantly reduced plasma T4. MEL or 5-MT (1-1,000 micrograms) were implanted in beeswax pellets in male hamsters exposed to short photoperiod (LD 10:14) to determine if either indole could prevent the short photoperiod-induced suppression of T4 (experiments 4 and 5). Regression and covariance analyses showed a significant log dose-related elevation of T4 in these hamsters, indicating equal potency of MEL and 5-MT in preventing the short photoperiod-induced suppression of T4. 5-MT given by either injection or implantation raised plasma triiodothyronine (T3) in female (experiment 2) but not in male hamsters.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Methoxytryptamine↗
PubMed1983

Natural and synthetic analogues of melatonin and related compounds. I. Effects on the reproductive system of the male Syrian hamster.

The potential antigonadotrophic properties of a number of synthetic and natural melatonin analogues were examined. Adult male hamsters received daily subcutaneous injections (25 micrograms/animal/day) each afternoon for 7 or 10 weeks. The only analogue to possess antigonadotrophic activity similar to melatonin wa 6-chloromelatonin; melatonin and 6-chloromelatonin produced a significant reduction in both testicular and accessory sex organ weights, plasma LH and pituitary prolactin. These results suggest that, in the Syrian hamster, 6-chloromelatonin acts as a melatonin agonist and may be of use to elucidate further the physiological role of melatonin.

Animals↗
PubMed1983

Natural and synthetic analogues of melatonin and related compounds. II. Effects on plasma thyroid hormones and cholesterol levels in male Syrian hamsters.

Synthetic or natural analogues of the pineal indole, melatonin, were injected separately every evening (1700 hours) for 7 (Exp. 1) or 10 (Exp. 2) weeks into adult male Syrian hamsters maintained in 14 hours of light and 10 hours of darkness each day. Plasma thyroxine (T4) levels were significantly depressed by 25 micrograms/day of melatonin (aMT) in both experiments. Injecting 25 micrograms/day either of acetyl methoxytryptophol or of synthetic analogues (hexanoyl methoxytryptamine, propionyl methoxytryptophol, or 6-chloro-melatonin) in Exp. 1 or of a natural analogue (N-acetylserotonin, 6-hydroxy-melatonin, hydroxytryptophol, or methoxytryptophol) in Exp. 2 had no effect on the circulating T4 levels. Plasma levels of triiodothyronine (T3) and thyrotropin (TSH) were unaffected in either experiment. Since none of the tested melatonin analogues is capable of suppressing circulating T4 concentration when given in a dose at which melatonin is reproducibly effective, the pineal-induced suppression of T4 is most likely mediated by melatonin. Plasma cholesterol levels were elevated only in hamsters receiving 6-chloromelatonin injections. However, plasma triglyceride levels were significantly higher than the diluent treated controls in Exp. 1 after injections of melatonin, acetyl methoxytryptophol, propionyl methoxytryptophol and 6-chloromelatonin. Interscapular brown adipose tissue was significantly heavier in melatonin treated animals.

Animals↗
PubMed1982

Occurrence of preantral intra-ovarian oocyte release in the rat and the Syrian hamster (Mesocricetus auratus).

Serial sections of 62 rat ovaries and 66 ovaries from Syrian hamsters (Mesocricetus auratus) have been examined by light microscopy. The relative and absolute percentage of preantral intra-ovarian oocyte release (PIOR) was recorded. PIOR was incomplete when the rupture of the basal membrane was not followed by the release of the oocyte from its preantral follicle into the ovarian stroma; conversely, a PIOR was regarded to be complete if the oocyte was released from the preantral follicle into the adjacent stroma. PIOR occurred in 23% of rat ovaries and in 32% of hamster ovaries; meiosis was not resumed. In rat ovaries the frequency of complete PIOR was 71%; the oocyte disappeared quickly after its release and herniated granulosa cells could invade capillaries. By contrast, incomplete PIOR was prevalent in hamster ovaries (67%); oocytes survived in the interstitial cortical tissue after the disappearance of their granulosa and theca cells ("free oocytes"). It is concluded that PIOR reflects the onset of follicular atresia and that different local factors are probably responsible for species differences.

Animals↗
PubMed1982

Chronic exposure to short photoperiod inhibits free thyroxine index and plasma levels of TSH, T4, triiodothyronine (T3) and cholesterol in female Syrian hamsters.

1. Chronic exposure of female Syrian hamsters (Mesocricetus auratus) for 9 weeks to a short photoperiod (10L:14D) depressed the pituitary-thyroid axis as indicated by a drop in circulating titers of thyroid stimulating hormone (TSH), thyroxine (T4), triiodothyronine (T3) and the free thyroxine index (FT4I) compared to animals maintained under long photoperiodic conditions (14L:10D). 2. Short day treatment also reduced plasma cholesterol levels. 3. Neither plasma triglycerides, glucose nor growth hormone (GH) levels differed between hamsters exposed to short or long daily photoperiods.

Animals↗
PubMed1982

Arginine vasotocin alters hamster pituitary LH and prolactin secretion in vitro.

1. Arginine vasotocin (AVT) stimulated the release of both luteinizing hormone (LH) and prolactin (PRL) from normal hamster (Mesocricetus auratus) anterior pituitary glands in a dose-dependent manner. 2. AVT significantly augmented the depressed level of PRL release from pituitaries of blind-pinealectomized animals. 3. LH release from the pituitaries of blinded hamsters was inhibited by AVT while FSH levels were unaffected.

Animals↗
PubMed1982

Effects of melatonin on thyroid physiology of female hamsters.

The effects of melatonin administration on thyroid physiology of female hamsters was investigated. A protocol of 25 micrograms given daily as subcutaneous injections late in the light period was found to inhibit blood levels of thyroxin (T4), triiodothyronine (T3) and thyrotropin (TSH). The free T4 index (FT4I) and the free T3 index (FT3I) were also significantly inhibited by melatonin injections. Decreasing the photoperiod under which the hamsters were kept, from 14 h light/10 h dark (14L/10D) to 10L/14D also resulted in decreased blood levels of these hormones. A protocol of melatonin injections using 2.5 mg daily, on the other hand, did not significantly inhibit blood levels of thyroid hormones or TSH; injection of this dose every afternoon into hamsters in long photoperiod significantly augmented the blood levels of T4. Continuously available melatonin in the form of subcutaneous implants of 1 mg melatonin in beeswax did not inhibit blood levels of thyroid hormones; furthermore, such implants prevented the inhibitory effects of injections of 25 micrograms melatonin. The results are consistent with the hypothesis that melatonin interferes with neurotransmitters which influence the synthesis or release of hypothalamic thyrotropin-releasing hormone.

Animals↗
PubMed1982

The effect of twice daily gonadotropin-releasing hormone (GnRH) administration and/or renal pituitary homografts on melatonin-induced gonadal atrophy in male Syrian hamsters.

Adult hamsters were maintained in a 14:10 light:dark cycle and divided into the following experimental groups: 1) controls, 2) daily afternoon (1600h) injections of 25 micrograms of melatonin (MEL), 3) daily MEL (1600) injections and twice daily (0900 and 1600h) injections of 1 microgram GnRH, 4) daily MEL (1600h) injections and two anterior pituitary glands implanted under the kidney capsule, and 5) MEL injections, GnRH injections and pituitary homografts. After 8 and 12 weeks of treatment, MEL injections resulted in testicular and accessory sex organ regression and depressed plasma PRL and testosterone. Twice daily injections of GnRH did not prevent atrophy of the testes and accessory sex organs in MEL-treated animals whereas animals with either homografts alone or the combination of GnRH injections and pituitaries implanted under the kidney capsule had normal or supranormal testicular and accessory sex organ sizes and plasma PRL and testosterone after 8 and 12 weeks of MEL treatment. The results suggest that prolactin alone can act to maintain normal testicular function in hamsters in which MEL-induced gonadal atrophy has taken place.

Animals↗
PubMed1982

Effects of blinding or afternoon melatonin injections on plasma cholesterol, triglycerides, glucose, TSH and thyroid hormone levels in male and female Syrian hamsters.

Adult male and female hamsters (Mesocricetus auratus) were divided into 3 treatment groups: intact, blinded, and intact hamsters which received daily sc injections of 25 microgram melatonin (MEL) at 1700 h. After 10 weeks of treatment, thyroxine (T4), triiodothyronine (T3) and cholesterol levels in both male and female blinded or MEL-treated animals were depressed compared to plasma titers of their respective intact control groups. Plasma thyrotropin (TSH) levels were depressed by MEL treatment but unaffected by blinding in male hamsters; in contrast, blinded or MEL-treated female hamsters had significantly higher TSH levels than the control group. Plasma triglyceride levels were depressed by both blinding and MEL-treatment in males but were unaffected in females.

Animals↗
PubMed1982

Thyroid hormones in male hamsters with activated pineals or melatonin treatment.

Blinding resulted in gonadal and prostatic atrophy and reduced plasma thyroxine (T4), free T4 index (FT4I) and reverse triiodothyronine (rT3) levels in adult male hamsters housed in light-to-dark, 14:10 h. Similar effects were seen after daily evening injections of 25 microgram melatonin. Pinealectomy prevented the effects of blinding or melatonin injections. There were no pineal- or melatonin-induced decrements in T3 or thyrotrophin (TSH) concentrations. TSH was elevated by blinding in one experiment but not in another, despite suppression of T4 and FT4I in both. Orally administered melatonin (approximately 245 microgram daily in drinking water through the evening and night) reduced the weight of testes and prostates and slightly lowered plasma T4 and FT4I, indicating the effectiveness of melatonin by this route. The capability of the pineal and of melatonin to suppress plasma T4 is not a result of sex-steroid-induced alteration of plasma binding but is most likely a result of variable suppression of the pituitary-thyroid axis at the level of TSH regulation and also at the level of T4 secretion and/or metabolism. Reduced rT3, but not T3 levels after blinding, may reflect the pineal-induced deficit in T4 as a substrate for rT3 formation, altered peripheral conversion of T4 or altered disposal of thyroid hormones.

Animals↗
PubMed1981

Influence of morning melatonin injections on the antigonadotrophic effects of afternoon melatonin administration in male and female hamsters.

Male and female hamsters were maintained on a long photoperiod (14L:10D) and were divided into the following groups: group 1, injected with vehicle at both 11.00 and 17.00 h; groups 2-5, injected with various doses of melatonin at 11.00 h (0, 100 micrograms, 500 micrograms, 1 mg) and 25 g of melatonin at 17.00 h. In males, a.m. vehicle p.m. melatonin treatments for 70 days led to atrophy of the testes and accessory sex organs and to a significant depression of both plasma LH and PRL titers. There was a graded inhibition of these actions of exogenous melatonin in animals receiving 100 micrograms, 500 micrograms or 1 mg a.m. injections of melatonin with 1 mg completely preventing the effects of p.m. melatonin. 80% of the female hamsters receiving daily p.m. injections of melatonin for 9 weeks became acyclic with significant decreases in uterine weight and increases in ovarian weight. All doses of melatonin given at 11.00 h suppressed the inhibitory actions of p.m. melatonin with the exception of vaginal cyclicity for which 500 micrograms or more was required to restore normal vaginal cyclicity in all animals. These results demonstrate that morning injections of melatonin can prevent the antigonadotropic effects of afternoon melatonin injections and provide support for the hypothesis that the paradoxical actions of melatonin may be related to the ability of the indole to regulate its own receptors.

Animals↗