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Biomedical subjects

A H Meier

Publications and source records attributed to A H Meier.

At least 55 records · Page 3Linked to original sources
PubMed1989

Resetting the annual cycle with timed daily injections of 5-hydroxytryptophan and L-dihydroxyphenylalanine in Syrian hamsters.

Daily injections of dihydroxyphenylalanine (DOPA), a precursor for dopamine synthesis, given 12 hr after daily injections of 5-hydroxytryptophan (5-HTP), a serotonin precursor, induced uterine growth and increased serum thyroxine and luteinizing hormone (LH) concentrations in scotosensitive female hamsters maintained on short daylengths. On the other hand, daily injections of DOPA given at the same time as daily injections of 5-HTP reduced uterine weights and serum concentrations of thyroxine and LH in scotorefractory female hamsters. These results indicate that the endogenous mechanism controlling seasonality (scotosensitivity and scotorefractoriness) in the Syrian hamster may be reset by drugs that influence serotonergic and catecholaminergic activity. This seasonal mechanism might involve two circadian systems that undergo seasonal changes in their phase relations.

5-Hydroxytryptophan↗
PubMed1987

Reduction of body fat stores by inhibition of prolactin secretion.

Reductions of prolactin secretion by bromocriptine treatment for 24 days reduced fat stores (abdominal and epididymal fat depots) in hamsters by 25-49% compared with control animals. However, body weights and food consumption were not affected. These results further substantiate an important role for prolactin in regulation of fat metabolism and indicate that bromocriptine might be used to decrease fat stores.

Adipose Tissue↗
PubMed1987

Circadian and seasonal variations of plasma insulin and cortisol concentrations in the Syrian hamster, Mesocricetus auratus.

Circadian rhythms of plasma insulin, cortisol, and glucose concentrations were examined in scotosensitive (reproductively sensitive to inhibitory effects of short daylengths) and scotorefractory male and female Syrian hamsters (Mesocricetus auratus) maintained on short (LD 10:14) and long (LD 14:10) daylengths. The baseline concentration (mean of all values obtained every 4 hr six times of day) of insulin was much greater in female than in male scotosensitive hamsters kept on short daylengths. These differences in insulin concentration may account for the observed heavy fat stores in female and low fat stores in male scotosensitive hamsters kept on short daylengths. The baseline concentrations of cortisol were approximately equal in both scotosensitive and scotorefractory males held on short and long daylengths, but were relatively low in females held on short daylengths and especially high in scotorefractory females held on long daylengths. The plasma concentrations of both cortisol and insulin varied throughout the day in many of the groups tested. However, the variations were not equivalent. The circadian variations of cortisol were similar irrespective of sex, seasonal condition and daylength. Peak concentrations generally occurred about 12 hr after light onset. In contrast, the circadian variations of insulin differed markedly. For example in male hamsters, robust daily variations were found in scotosensitive hamsters held on short daylengths but not on long daylengths and in scotorefractory hamsters held on long daylengths but not on short daylengths. Furthermore, the daily peak occurred during the light in the scotosensitive hamsters and during the dark in the scotorefractory animals. Neither the daily feeding pattern (about 60% consumed during dark) nor the daily variations of glucose concentration varied appreciably with seasonal condition or daylength. They do not appear to determine nor directly reflect the variations in cortisol and glucose concentrations. It is postulated that the daily rhythms of cortisol and insulin are regulated by different neural pacemaker systems and that changes in the phase relations of circadian systems account in part for seasonal changes in body fat stores.

Animals↗
PubMed1985

Fattening responses to daily intervals of heat exposure in the Syrian hamster.

Warm ambient temperature (38 degrees C) provided daily for one hr induced time-dependent changes in body weight and fat stores in the Syrian hamster. In animals held on 10-hr daily photoperiods and room temperature (23 degrees C), daily one-hour thermopulses at 8 and 20 hr after light onset stimulated increases in body weights and indices of body fat storage. Abdominal fat pad weights of these groups were twice those of untreated controls after 17 and 28 days of thermoperiodic treatments. On the other hand, daily thermopulses were completely ineffective at 0 and 16 hr after light onset. These results demonstrate that body fat stores may be influenced by a temporal interaction of environmental stimuli and implicate underlying circadian mechanisms in the regulation of body fat.

Adipose Tissue↗
PubMed1985

Circadian and seasonal variations of lipogenesis in the Gulf killifish, Fundulus grandis.

Lipogenesis was measured in male Gulf killifish at three seasons under different environmental conditions. Cold temperature is stimulatory for lipogenesis in the fall and spring, but warm temperature is more stimulatory during the summer. The highest rate of lipogenesis occurs during the spring. However, total body fat content is relatively low at that time, which suggests that lipids are being actively mobilized as well. A high rate of lipogenesis also occurs at a cold temperature (20 degrees C) during the fall, and more lipids are stored at that time of year, as evidenced by higher liver and body fat stores. Both lipogenesis and total body fat content are at a minimum during the summer. These results indicate that high levels of lipid synthesis do not necessarily result in higher body fat content, as the latter is the product of both lipid synthesis and lipid mobilization. This study is also in agreement with previous studies which had indicated a changing responsiveness to temperature during the annual cycle of the Gulf killfish.

Animals↗
PubMed1985

Prolactin permits the expression of a circadian variation in lipogenic responsiveness to insulin in hepatocytes of the golden hamster (Mesocricetus auratus).

Lipogenesis was determined at two times (07.00 and 16.00 h) during a 14-h daily photoperiod (08.00-22.00 h) in freshly prepared hamster hepatocytes with or without addition of insulin. The hamsters were pretreated for 5 days with bromocriptine (to inhibit prolactin secretion), bromocriptine and prolactin replacement, or control saline injections. Lipogenesis was determined by incorporation of [14C]acetate into total cell lipids over a 30-min interval. Lipogenesis was three times greater at 07.00 than at 16.00 h and insulin was effective in stimulating further lipogenesis only at 07.00 h. Bromocriptine pretreatment severely reduced incorporation of radiolabel at 07.00 h to levels comparable with controls at 16.00 h and completely inhibited the stimulatory effect of insulin at 07.00 h. Prolactin replacement in bromocriptine-treated hamsters reversed the inhibitory effect of bromocriptine on hepatocyte lipogenesis and promoted dramatic lipogenic responses to insulin at 07.00 h. These results indicate that insulin stimulates hepatic lipogenesis only during some portion of a day and that prolactin facilitates the lipogenic response.

Animals↗
PubMed1985

Prolactin permits the expression of a circadian variation in insulin receptor profile in hepatocytes of the golden hamster (Mesocricetus auratus).

Insulin receptor profile was determined in freshly isolated hamster hepatocytes at two times (07.00 and 16.00 h) during a 14-h daily photoperiod (08.00-22.00 h). The hamsters were pretreated for 5 days with bromocriptine (to inhibit prolactin release), bromocriptine and prolactin replacement, or control saline injections. The insulin receptor profile was determined by Scatchard plot analysis. The insulin receptor number (high and low affinity) was three times greater at 07.00 than at 16.00 h among saline-injected controls. However, their affinities did not differ. Bromocriptine pretreatment reduced (70%) both the high and low affinity receptor numbers and increased the affinity of the high affinity receptor at 07.00 h. Prolactin replacement in bromocriptine-treated hamsters restored the receptor profile at 07.00 h to control values. These data indicate that prolactin facilitates the expression of a circadian variation in insulin receptor profile.

Animals↗
PubMed1984

Circadian rhythms of lipogenic and hypoglycaemic responses to insulin in the golden hamster (Mesocricetus auratus).

Lipogenesis and blood glucose concentrations were determined at 4-hourly intervals in control and insulin-treated golden hamsters maintained on 14-h daily photoperiods (08.00-22.00 h). Lipogenesis was studied by measuring the incorporation of label into liver and fat pad lipids in animals killed 30 min after i.p. [3H]acetate injection and 2 h after insulin or saline (control) injections. Circadian rhythms of lipogenesis and plasma glucose concentration were present in both control and insulin-treated hamsters. In control animals most lipogenic activity occurred during the dark period and early during the daily photoperiod (14 h light: 10 h darkness). There were dramatic differences in the lipogenic (fat pad) and hypoglycaemic responses to insulin which varied as a function of the time of day at which insulin was injected. Insulin stimulated fivefold increases in lipid deposition (fat pad incorporation) when injected late during the dark period but had little or no effect 4-8 h after the onset of light. Daily injections for 8 days also produced variable cumulative effects on body fat stores as a function of the time of day. Insulin injected late during the dark period stimulated a 40% increase in abdominal fat weight over controls, whereas insulin injected at 4 and 12 h after the onset of light had no effect on abdominal fat weight. Insulin decreased plasma glucose concentrations markedly at 8 and 20-24 h after the onset of light but had no apparent hypoglycaemic activity (120 min after its injection) at 4 h after the onset of light.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetates↗
PubMed1983

Tryptophan feeding adversely influences pregnancy.

Additional tryptophan during pregnancy reduces embryo and neonate survival in the golden hamster, Mesocricetus auratus. Relatively small doses of exogenous serotonin have been reported to cause abortions in several vertebrate species (1, 2, 3, 4, 5). Smaller doses reduce litter sizes, increase still births and neonate abnormalities, and otherwise influence pregnancy adversely. These effects are produced by serotonin throughout pregnancy, beginning at implantation (6). The availability of tryptophan is probably the most important rate limiting factor in serotonin synthesis (7). Inasmuch as tryptophan is an essential amino acid and is not synthesized by the body, the diet is the sole source; studies have shown that increases (8) or decreases (9) in dietary tryptophan lead to concomitant changes in serotonin content. Because tryptophan is employed in humans to promote sleep (10, 11, 12) and to decrease appetite (13) we felt it might be important to test whether increased amounts of diet tryptophan can adversely influence pregnancy.

Animals↗
PubMed1983

Tryptophan feeding induces sensitivity to short daylengths in photorefractory hamsters.

The annual cycle of reproduction in the golden hamster (Mesocricetus auratus) is characterized by gonadal regression in response to decreasing daylengths in fall. However, hamsters become refractory to the inhibitory effects of short daylengths in late winter when gonadal recrudescense occurs. Our results show that addition of tryptophan to the diet of photorefractory hamsters causes them to respond to short daylengths in a manner similar to photosensitive hamsters in winter. Similar tryptophan treatment did not influence testes weights and fat stores of hamsters maintained on long daylengths.

Animals↗
PubMed1982

Adrenal innervation may be an extrapituitary mechanism able to regulate adrenocortical rhythmicity in rats.

The daily rhythm of plasma corticosteroid concentration was studied in nonhypophysectomized rats and in hypophysectomized rats implanted with beeswax pellets containing ACTH and T4. In the first experiment, male hypophysectomized rats were placed on a 12-h light, 12-h dark cycle (onset of light, 0600 h) with food and water available ad libitum. Beeswax pellets containing 1.5 mg ACTH and 150 micrograms T4 were implanted sc in these rats. Beginning 4 days after the implantation of ACTH and T4, daily rhythms of plasma corticosteroid concentration were detected in these rats on 3 successive days. The injection of sodium pentobarbital (40 mg/kg BW) or atropine sulfate (10 mg/kg BW) ip at 1200 h blocked the expected rise in plasma corticosteroid concentration at 1800 h in both these rats and a group of intact rats. In the second experiment, adrenal innervation was disrupted by spinal cord transection at the T-7 level. L-1-transected rats served as operated controls, and a third group was maintained as unoperated controls. One week after surgery, daily rhythms of plasma corticosteroid concentration were present in both unoperated and L-1 controls (P less than 0.01) but not in the T-7-transected rats. Inasmuch as an extrapituitary mechanism was capable of maintaining adrenocortical rhythmicity in hypophysectomized rats and disruption of adrenal innervation suppressed adrenocortical rhythmicity, it was hypothesized that adrenal innervation may be an extrapituitary mechanism which has a role in adrenocortical rhythmicity.

Adrenal Cortex Hormones↗
PubMed1979

Circadian rhythms of plasma corticosterone binding activity in the rat and the mouse.

Corticosterone binding activity (CBA) and corticosteroids were measured by competitive protein binding techniques in plasma samples drawn from rats and mice at different times of day. Circacian rhythms of plasma CBA were found in both rats (onset of 14-h daily photoperiod: 0600) and mice (onset of 14-h daily photoperiod: 0700). The plasma CBA rhythm was bimodal in rats with peaks at 1000 and 1800 and unimodal in mice with peak level from 0100 to 0500. Circadian rhythms of plasma corticosteroid concentration with peaks during the late afternoon were confirmed in both rats and mice. The plasma CBA rhythms appear to be related to the circadian rhythms of both locomotor activity and plasma corticosteroid concentration.

Adrenal Cortex Hormones↗
PubMed1979

Circadian phase-dependent prolactin mechanisms in hepatic lipogenesis of a teleost.

In the teleost, Fundulus grandis, injections of prolactin early in the light phase cause an immediate 50% depression in the rate of hepatic lipogenesis ([14C]acetate incorporation); 10 h later, that rate has returned to levels not different from controls. Injections of prolactin late in the light phase cause an even more dramatic immediate depression of lipogenesis (79%) followed by a gradual increase in lipogenic rate which is 2.6 times higher than the control rate after 24 h. The stimulation of lipogenesis by prolactin is blocked by simultaneous treatment with indomethacin, an inhibitor of prostaglandin synthesis. These circadian phase-dependent effects of prolactin on hepatic lipogenesis are discussed with reference to possible mechanisms of action exerted by endogenous prolactin rhythms.

Acetates↗