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Biomedical subjects

B Malpaux

Publications and source records attributed to B Malpaux.

At least 55 records · Page 3Linked to original sources
PubMed1997

Control of the circannual rhythm of reproduction by melatonin in the ewe.

Annual variations in day length are responsible for seasonal changes in reproductive activity in sheep. However, in constant photoperiodic conditions, ewes express an endogenous rhythm characterized by alternations of reproductive activity and quiescence that are not synchronized among animals. Thus, the main role of photoperiod in the natural environment appears to be the synchronization of this endogenous rhythm. Photoperiodic information is processed through a complex nervous and endocrine pathway to modulate reproductive activity. Light information perceived at the level of the retina is transformed through neural processing into an endocrine signal by the pineal gland: the nocturnal increase in melatonin release. Recent studies strongly suggest that melatonin has a hypothalamic target to modulate the reproductive neuroendocrine axis. Most LHRH perikarya are located in the preoptic area, but this region is devoid of melatonin receptors, and microimplants of melatonin placed in the preoptic area do not effect LHRH release. Thus, melatonin influences LHRH neurones indirectly and must involve interneurons. Good evidence now exists to demonstrate that a population of dopaminergic neurons with axons projecting to the median eminence is one of these interneurons.

Animals↗
PubMed1997

The characteristics of the melatonin secretory rhythm are not modified by the stage of pregnancy in ewes.

An experiment was designed to study if, in the same animals, characteristics of the plasma melatonin rhythm vary during pregnancy in ewes. Thirteen Ile-de-France ewes were maintained in natural photoperiod and measurements of the characteristics of the rhythm of melatonin secretion were determined during one estrous cycle and then during pregnancy. Duration and mean plasma concentrations of the nocturnal melatonin elevation were measured at four stages of pregnancy. Melatonin concentrations during the elevation were not significantly affected by the stage of the estrous cycle (mean +/- SEM: 227.8 +/- 34.9, 263.2 +/- 24.2 and 232.4 +/- 27.1 pg/mL for follicular phase, early luteal phase and late luteal phase, respectively), or by the stage of pregnancy (292.8 +/- 22.2, 268.7 +/- 21.7, 267.4 +/- 27.9 and 258.3 +/- 23.5 pg/mL for first, second, third and fourth month of pregnancy, respectively). A strong individual effect was detected (P < 0.01) for melatonin concentrations during elevation (range: 119.2 +/- 11.8 to 396.6 +/- 26.0 pg/mL of plasma). A highly significant correlation coefficient was observed within individuals for night melatonin concentrations between the different physiological stages. It was concluded that melatonin secretion is unaffected by the stage of pregnancy.

Animals↗
PubMed1997

Estradiol acts locally within the retrochiasmatic area to inhibit pulsatile luteinizing-hormone release in the female sheep during anestrus.

In the present study we have identified a site of action of estradiol in the inhibition of LH secretion during anestrus in the ewe. In the first experiment, we studied six sites: the medial preoptic area, the lateral preoptic area, the ventromedial hypothalamus, the ventrolateral hypothalamus, the retrochiasmatic area (RCh), and the periventricular posterior hypothalamus. We compared the changes in parameters of pulsatile LH secretion (interpulse interval, mean nadir, mean amplitude, and mean area under curve) during three 6-h sampling periods: before and 30-36 h and 9 days after intracerebral implantation of crystalline estradiol. Animals that received estradiol in the RCh (n = 5) showed a significantly greater increase in both the intervals between pulses of LH (up 116%, p < 0.03) and the area under the curve (up 180%, p < 0.01) than any of the other groups of 7 animals. In the second experiment, implantation of estradiol in the RCh (n = 6) induced an increase in the intervals between pulses of LH (p < 0.03), whereas receiving an empty implant (n = 6) had no effect, showing that estradiol specifically induced increases in the intervals between pulses. Thus, estradiol appears to act in the RCh where the dopaminergic A15 nucleus, known to inhibit pulsatile LH release, is located.

Anestrus↗
PubMed1997

Characterization of the short day-induced decrease in median eminence tyrosine hydroxylase activity in the ewe: temporal relationship to the changes in luteinizing hormone and prolactin secretion and short day-like effect of melatonin.

In the ewe, photoperiod modulates LH and PRL secretion as well as median eminence (ME) dopaminergic activity. The studies reported here were designed to characterize the functional significance of this photoperiodic modulation of ME dopaminergic neuron activity in relation to the regulation of LH and PRL secretion. The aim of the first experiment was to assess whether photoperiodic changes in hypothalamic dopaminergic activity were temporally linked to changes in either PRL or LH secretion. The purpose of the second experiment was to determine whether melatonin mimicked the effects of photoperiod on ME dopaminergic activity. In the first experiment, LH and PRL secretion, hypothalamic tyrosine hydroxylase (TH) activity, and catecholamine contents were determined in ovariectomized estradiol-treated ewes either during long days (LD; control group) or after 5, 25, and 76 short days (SD). SD were associated with a stimulation of LH secretion and a decrease in ME TH activity, which were both expressed only in the 76 SD group. In contrast, the SD-induced inhibition of PRL secretion was already maximal in the 25 SD group. In the second experiment, LH secretion and hypothalamic dopaminergic activity were studied in ovariectomized estradiol-treated ewes kept in LD and then treated for 0 (control), 25, or 77 days with melatonin implants producing a SD-like effect on LH secretion. Melatonin induced a decrease in PRL secretion (observed after 25 days of treatment), as well as a stimulation of LH secretion and a decrease in ME TH activity and dopamine content (observed only after 77 days of treatment). In conclusion, the decrease in ME dopaminergic activity associated with SD exposure or the SD-like effect of melatonin appears unrelated to the regulation of PRL secretion. The SD-like effect of melatonin on ME dopaminergic activity suggests that melatonin mediates the effect of SD on this activity. The regulation of ME dopaminergic activity can thus be considered a probable step in the photoperiodic regulation of LH secretion.

Animals↗
PubMed1997

Does a short loop feedback mechanism for the control of luteinizing hormone secretion exist in the ewe?

It is not known whether a short loop feedback mechanism for the regulation of LH exists in sheep. This study on ovariectomized ewes investigated whether a bolus injection (10, 1, and 0.1 microg LH or 1 microg BSA; n 4) or a 3-h continuous infusion of exogenous LH (100 or 1 ng/min; n = 7) into the third ventricle through a permanent indwelling cannula could influence the activity of the GnRH pulse generator, as determined by measurement of endogenous LH secretion. To assess the potential for involvement in a LH short loop feedback system and to estimate the level of LH in the hypothalamic milieu, the concentrations of LH in the peripheral circulation, portal circulation, and third ventricle were measured during an estradiol-induced preovulatory LH surge (n = 4). Neither the bolus nor continuous administration of LH into the third ventricle had any effect on the mean interpulse interval, nadir, pulse amplitude, or circulating level of systemic LH. Furthermore, despite portal LH concentrations being more than 20-fold higher than jugular LH concentrations, LH levels in third ventricular cerebrospinal fluid remained barely detectable and did not reflect dynamic secretory events in the peripheral or hypothalamo-hypophyseal portal blood. These data demonstrate that in ewes, little pituitary LH reaches the third ventricle, and the small amount that does is unable to affect peripheral gonadotropin release. Our study suggests, therefore, that a short loop feedback system for LH does not exist in the ewe.

Animals↗
PubMed1997

Simultaneous measurement of gonadotropin-releasing hormone in the third ventricular cerebrospinal fluid and hypophyseal portal blood of the ewe.

GnRH is present in the hypophyseal portal blood and cerebrospinal fluid (CSF) of several species investigated, including sheep, but the precise relationship between these two compartments of GnRH is unknown. In the present study, ovariectomized steroid-treated ewes were surgically prepared for the simultaneous collection of portal blood and third ventricular CSF. Ten-minute samples were collected for pulse analysis after progesterone removal and hourly for comparisons during the estradiol-induced LH surge. The time of onset of the portal (15.3 +/- 0.5 h after estradiol) and CSF (15.9 +/- 0.2 h) GnRH surges was similar and occurred coincidentally with the LH surge (15.6 +/- 0.4 h). The period of the surge during which GnRH concentrations exceeded half-maximal levels (portal, 7.3 +/- 1.5 h; CSF, 7.3 +/- 0.3 h) was the same and outlasted the corresponding LH surge period (3.3 +/- 0.3 h). LH pulses started and peaked later than the corresponding portal GnRH pulses (onset difference, 10 +/- 1 min; peak difference, 16 +/- 1 min; P < 0.01 for both), but the times of pulse onset and peak were not significantly different from those of concomitant CSF GnRH pulses (onset difference, 8 +/- 6 min; peak difference, 8 +/- 4 min). Although the times of pulse onset and peak did not differ between the portal and CSF GnRH compartments (onset difference, 4 +/- 6 min; peak difference, 6 +/- 2 min), CSF GnRH pulses were longer than their portal counterparts (CSF, 38 +/- 3 min; portal, 15 +/- 1 min; P < 0.01). The amplitude of jugular LH pulses was strongly correlated (r2 = 0.85) with portal GnRH pulse amplitude, but not with that of CSF GnRH pulses (r2 = 0.45); there was no correlation between portal and CSF GnRH pulse amplitudes (r2 = 0.25). These data show that third ventricular CSF GnRH reliably relates neurosecretory events occurring within the hypophyseal portal system at the time of the preovulatory LH surge, but is not as precise as portal GnRH in marking a LH pulse.

Animals↗
PubMed1996

Photoperiodic modulation of monoamines and amino-acids involved in the control of prolactin and LH secretion in the ewe: evidence for a regulation of tyrosine hydroxylase activity.

Several neurotransmitters are implicated in the photoperiodic regulation of prolactin and luteinising hormone (LH) secretion in the ewe. This work investigated whether catecholamines, gamma-amino butyric acid (GABA), excitatory amino acids and serotonin diencephalic contents are affected by photoperiod and how such changes relate to the seasonal effects of photoperiod on LH and prolactin secretions. Moreover, to determine whether photoperiod can influence catecholamine biosynthesis, the activity of its rate limiting enzyme, tyrosine hydroxylase (TH) was also investigated. TH activity and the tissue content of the monoamines and their metabolites were measured in stalk-median eminence (SME), preoptic area (POA) and the mediobasal, mediodorsal and laterobasal aspects of the hypothalamus. Investigation of excitatory amino acids and GABA was limited to the POA and the SME. Ovariectomized ewes were initially maintained in long days (LD) for 70 days. Thereafter half the ewes remained exposed to long days and the other half were transferred onto short days (SD) for 63 to 66 days to induce a stimulation of LH secretion and an inhibition of prolactin secretion. In each photoperiodic regime, half the ewes were treated with a subcutaneous oestradiol implant (+E) and half were not (-E). As expected, short days induced a decrease in prolactin and an increase in pulsatile LH secretion. These neuroendocrine changes were associated with a decrease in the TH activity of the SME in both oestradiol treated and non treated animals (146.5 +/- 24.1, 167.6 +/- 26.5 U TH/g of tissue in LD-E and LD+E vs 83.5 +/- 12.4 and 95.0 +/- 30.2 U TH/g of tissue in SD-E and SD+E animals; P < or = 0.01). A similar and parallel short day-induced decrease was observed in the tissue content of dopamine and its metabolite, 3,4-dihydroxy-phenylacetic acid (SD level were 55% of LD levels, P < 0.05). In POA, a short day-induced decrease in dopamine (18%; P < or = 0.05) and GABA (16.4%; P < or = 0.05) content and an oestradiol-induced decrease in aspartate (15.6%; P < or = 0.05) content were found. This study provides the first report of a photoperiodic control of the synthesis activity of catecholaminergic neurones of the SME in the ewe. The photoperiod-induced changes in dopaminergic activity at the level of the SME were associated with changes in LH and prolactin secretion indicating that TH activity of dopaminergic neurones of the SME could be a critical component of the photoperiodic regulation of LH and/or prolactin secretion. In particular, this finding is in agreement with the hypothesis that photoperiod can control a dopaminergic pathway inhibitory of LH secretion and which ends in the median eminence.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1996

Characteristics of the plasma melatonin rhythm are not modified by steroids during the estrous cycle in IIe-de-France ewes.

Two experiments were designed to determine whether gonadal steroids during the estrous cycle may modify the characteristics of the plasma melatonin rhythm. In the first experiment, 12 ovariectomized estradiol-treated ewes were used and exposed to constant short days. The experimental design was a latin square to distinguish between steroid treatments and individual effects on melatonin secretion. Twenty four hours before the bleeding period (hourly during 20 hr) and with a 1 week interval, animals were treated with a) additional subcutaneous estradiol implants, b) progesterone devices, or c) control. In the second experiment, nine ewes received a treatment combining fluorogestone acetate devices and pregnant mare serum gonadotrophin to induce synchronous ovulations. Samples for melatonin determination were obtained hourly for 13 hr at three stages of estrous cycle: follicular phase, early luteal phase, and late luteal phase. Ovarian activity was monitored by taking daily samples for progesterone analysis and ovulation rate was determined by laparoscopy. Duration and mean melatonin plasma concentrations of the elevation were calculated for each ewe and each night and analysed by latin square test (experiment 1) or ANOVA (experiment 2). Melatonin concentrations of elevation and duration of elevation were not significantly affected by hormonal treatments or by phase of estrous cycle. A strong individual effect was detected (P < 0.01) for both parameters in both experiments. It was concluded that melatonin secretion is unaffected by steroid administration or by phase of estrous cycle. The existence of very high inter-individual variation suggest that both parameters are individual characteristics of each animal which may have a strong genetic basis.

Animals↗
PubMed1996

Initiation of the oestradiol-induced inhibition of pulsatile LH secretion in ewes under long days: comparison of peripheral versus central treatment and neurochemical correlates.

In the ewe, the inhibition of pulsatile LH secretion by oestradiol during long days depends on dopaminergic activity and could involve amino acid transmitters. In the first experiment of the present study we observed the changes in LH secretion in ovariectomised ewes under long days immediately after subcutaneous implantation of oestradiol (peripheral treatment). In the second experiment, in order to identify the site of action of oestradiol, we observed the LH changes following intracerebral infusion of oestradiol through a microdialysis membrane (central treatment) within the preoptic area, the mediobasal hypothalamus (MBH) or the retrochiasmatic area (RCh) and measured amino acids and catecholaminergic transmitters and metabolites within the dialysates. With peripheral treatment, the amplitude, the nadir and the area under the LH pulse curve decreased within 4 to 8 h of the insertion of a subcutaneous oestradiol implant. After 18 h, the amplitude and the area under the pulses increased, as well as the intervals between pulses (from 49.9 + 1.4 min to 75.6 +/- 5.9 min). With central oestradiol treatment. LH changes were similar whatever the site of oestradiol infusion, suggesting either multiple sites of action or diffusion between structures. Twenty hours after the beginning of intracerebral oestradiol treatment, the amplitude and the area under the pulses increased, as did the interval between LH pulses (from 49.5 +/- 4.1 min to 73.2 +/- 14.2 min). Comparison of peripheral with central oestradiol treatment suggested that the long-lasting decrease in the nadir, as well as the transitory decrease in the amplitude and area, before 18 h in experiment 1 are reflections of hypophysial effects. In contrast, the increases in amplitude and area under the LH pulse curve seen 18-20 h after oestradiol in the two experiments could be due to the higher amplitude of LHRH pulses, as a result of an early stimulatory effect of oestradiol. After central oestradiol infusion, there was a decline in the concentration in the dialysate of two metabolites of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the RCh, suggesting an early inhibition of monoamine oxidase by the steroid. During the inhibition of LH pulsatility the concentration of gamma-aminobutyric acid in the dialysate from the RCh and the MBH increased, suggesting the participation of this transmitter in the changes induced by oestradiol under long days.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1995

Effect of hypothalamic infusion of a dopamine D1 receptor antagonist on prolactin secretion in the ewe.

In this study we investigated whether dopamine D1 receptors in the hypothalamus are involved in the control of prolactin secretion in ovariectomised, oestradiol implanted ewes. The D1 antagonist SCH23390 or vehicle was infused into either the preoptic area (POA) or the ventromedial hypothalamus (VMH). During infusion into the VMH, prolactin concentrations declined significantly and did not return to control values until more than 60 min after the infusions had stopped. In contrast, infusion into the POA had no significant effect. These results are in accord with the hypothesis that dopaminergic pathways within the hypothalamus stimulate prolactin secretion via dopamine D1 receptors in the VMH.

Animals↗
PubMed1995

Regulation of luteinizing hormone-releasing hormone (LHRH) secretion by melatonin in the ewe. I. Simultaneous delayed increase in LHRH and luteinizing hormone pulsatile secretion.

This study was performed to test the hypothesis that a short-day-like melatonin treatment would induce simultaneous stimulations of LHRH and LH secretion. Simultaneous serial samples of jugular and portal blood to assay LH and LHRH, respectively, were carried out at three different periods according to the expected changes in LH secretion. Twenty-six ovariectomized ewes bearing a subcutaneous implant of estradiol and exposed to a long-day photoperiod (16L:8D) received s.c. implants of melatonin on Day 0. LHRH and LH pulsatile secretion was studied in the first group during inhibition of LH secretion by long days (on Day -1, n = 6). A second group (n = 9) was sampled on Day 39. Moreover, seven animals from this group were sampled on Day 46 to focus on the beginning of LH increase. The third group (n = 6) was sampled during maximal LH secretion (Day 74, n = 6). A fourth group (n = 5) composed of noncannulated animals was sampled for LH secretion only as a control for the effects of surgical procedures. On each occasion, blood was collected every 10 min for 6 h. Between Day -1 and Day 39, LHRH pulse frequency was low and did not differ. In the same animals, this value became 2.5-fold higher between Day 39 and Day 46. Finally, on Day 74, LHRH pulse frequency was about 6-fold higher than on Day -1.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Regulation of luteinizing hormone-releasing hormone (LHRH) secretion by melatonin in the ewe. II. Changes in N-methyl-D,L-aspartic acid-induced LHRH release during the stimulation of luteinizing hormone secretion by melatonin.

Injection of N-methyl-D,L-aspartic acid (NMDA), a glutamatergic agonist, cause an acute release of LH. The amount of LH released is modulated by photoperiod in sheep; it is larger in animals in which reproductive activity has been inhibited by long days than in those photostimulated by short days. The purpose of the present experiment was to determine whether this effect of photoperiod on LH response to NMDA is mediated by melatonin and whether it reflects a change in the amount of LHRH released in response to NMDA. Twenty-one ovariectomized ewes, bearing an s.c. implant of estradiol and kept in long days (16L:8D), received s.c. implants of melatonin on Day 0. The effect of a single injection of NMDA (5 mg/kg i.v.) on LHRH and simultaneous LH secretion was studied at three different periods of the stimulation of LH secretion by the melatonin treatment: first, during inhibition of LH secretion by long days (on Day -1, n = 6); secondly, during the onset of the LH increase (on Day 39, n = 9); and thirdly, during maximum LH secretion (on Day 74, n = 6). The stimulation of LHRH secretion by administration of NMDA was characterized by a large and long increase in LHRH concentration on Day -1 and Day 39. There was no significant difference between these two periods in terms of duration and amplitude of the LHRH release. On Day 74, the stimulation was much shorter than on Day -1 and Day 39 and was followed by a phase of decreased pulsatility of LHRH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

The ovine pars tuberalis does not appear to be targeted by melatonin to modulate luteinizing hormone secretion, but may be important for prolactin release.

The pineal hormone, melatonin, transduces photoperiodic information to the neuroendocrine axis of seasonally breeding mammals to regulate reproduction. It is not known where or how melatonin achieves this effect, but the recent identification of the pars tuberalis (PT) as the area with the highest density of melatonin binding sites suggests that this pituitary subdivision may be an important target for the actions of this indoleamine on luteinizing hormone (LH) and prolactin release. The present study was designed to test this hypothesis. Ovariectomized oestradiol-implanted ewes were exposed to inhibitory long days for 85 days and then received melatonin micro-implants (Day 0) in the mediobasal hypothalamus (MBH; n = 7) or PT (Melatonin-PT; n = 5). The effect of these micro-implants was compared to ewes receiving empty micro-implants in the PT (Sham-PT; n = 5). For LH, bi-weekly jugular blood samples were collected and for prolactin, samples were collected every 20 min for 5 h, with the first hour discarded, on Days -4, 26 and 69. Melatonin implanted in the MBH stimulated LH secretion in 3 ewes by Day 46 +/- 0 after implantation, and one ewe by Day 67 after implantation. In contrast, no Melatonin-PT or Sham-PT ewes exhibited an increase in LH secretion by the end of the study (Day 70). A subsequent experiment, in which the Sham-PT ewes were implanted with melatonin both subcutaneously and in the PT showed that the micro-implants did not impair the ability of the ovine reproductive neuroendocrine axis to respond to melatonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗
PubMed1995

Luteinizing hormone (LH)-releasing hormone in third ventricular cerebrospinal fluid of the ewe: correlation with LH pulses and the LH surge.

Morphological evidence suggests that LHRH may be secreted into cerebrospinal fluid (CSF), but only in the rhesus monkey has CSF LHRH been found to reflect changes occurring in the LHRH neuroendocrine system. This study investigated whether LHRH is detectable in ovine CSF and, if so, whether its release profile correlates with peripheral LH profiles during pulse and surge conditions. A polyethylene catheter was threaded through a stainless steel guide cannula previously implanted into the third ventricle of an ovariectomized ewe, which enabled continuous CSF withdrawal on repeated occasions. The first experiment (n = 3) showed that peripheral LH concentrations were unaffected by CSF removal at rates of 12, 30, 50, and 100 microliters/min, and the second (n = 4) established that CSF LHRH secretion was pulsatile, with considerable variation in pulse amplitude (6.3 +/- 1.8 pg/ml; range, 1.3-18.7 pg/ml). In the third experiment (n = 6), an endogenous LH surge was induced after progesterone withdrawal and 17 beta-estradiol administration. Although CSF LHRH (15.3 +/- 1.3 h) and peripheral LH (14.8 +/- 1.0 h) surges occurred simultaneously, CSF LHRH concentrations were greater than half-maximal levels for longer (11.0 +/- 0.6 h; P < 0.005) than LH concentrations (6.0 +/- 0.4 h). This is the first study in sheep to reveal the presence of LHRH in CSF and show that it expresses dynamic and longer term changes coincident with peripheral LH fluctuations. CSF LHRH analysis also permits repeated sampling from individuals and, therefore, long term within-individual comparisons.

Animals↗
PubMed1995

Dopaminergic control of LH secretion by the A15 nucleus in anoestrous ewes.

Annual variations in the secretion of LH are responsible for seasonal changes in ovulatory activity in ewes. This hormonal pattern reflects an increase in the intensity of the negative feedback exerted by oestradiol under long days. Neuropharmacological studies have shown that this inhibition of LH secretion involves activation of catecholaminergic systems from preoptic and mediobasal hypothalamus (MBH) by oestradiol during anoestrus, and that 5-hydroxytryptamine inputs may also play a role. Within the MBH, the most important structures appear to be the retrochiasmatic region of the hypothalamus, which contains the A15 dopaminergic nucleus, and the median eminence, which contains the axon terminals of the GnRH cells controlling the pulsatile release of LH. In ovariectomized ewes in which oestradiol tonically inhibits LH secretion during the anoestrous season, LH pulse frequency is increased when the cells of the A15 nucleus are destroyed. The median eminence and other mediobasal structures contain more catecholamines and their metabolites under long days than under short days. Microdialysis of the A15 nucleus in vivo during long days revealed increased catecholaminergic activity under oestradiol treatment due to stimulation of tyrosine hydroxylase, the rate-limiting enzyme in the pathway of catecholaminergic synthesis. Tyrosine hydroxylase activity within the median eminence is increased under the various photoperiodic regimens that inhibit LH secretion. Neurochemical changes in the A15 nucleus and median eminence, in response to photoperiodic or oestradiol treatments, suggest a functional relationship which acts at the level of the GnRH axon terminals.

Anestrus↗
PubMed1994

Estradiol increases tyrosine hydroxylase activity of the A15 nucleus dopaminergic neurons during long days in the ewe.

In the ewe, the inhibition of LH secretion during long days results from increased negative feedback by estradiol. This effect depends on aminergic systems, mainly in the lateral retrochiasmatic area, a region that includes the dopaminergic A15 nucleus. Recently we demonstrated that the inhibition of LH secretion by estradiol under long days is accompanied by an increase in extracellular levels of amine metabolites (3,4-dihydroxyphenylacetic acid [DOPAC], homovanillic acid [HVA], and 5-hydroxyindolacetic acid [5-HIAA]) in the lateral retrochiasmatic area of the hypothalamus. The increase in catecholamine metabolite levels could be due to stimulation of tyrosine hydroxylase (TH) activity in the catecholaminergic systems of this region. In the present study, we tested this hypothesis by incorporating in vivo measurement of TH activity in the brain into our microdialysis model. We used this approach to first examine the effect of estradiol on TH activity in both the lateral retrochiasmatic area and the caudate nucleus during long days (experiment 1). Two dialysis sessions were carried out on each of six ewes: one after a 10-day estradiol treatment (s.c. implant) and the other after 10 days without estradiol treatment. Estradiol treatment reduced pulsatile LH secretion without affecting the secretion of prolactin. The steroid also significantly increased TH activity in the lateral retrochiasmatic area as assessed by the formation of L-3-4 dihydroxyphenylalanine (L-DOPA). There was no variation in the TH activity of the catecholaminergic systems of the caudate nucleus, suggesting that the effect of estradiol is specifically hypothalamic. In experiment 2, we estimated the relative contribution to the accumulation of L-DOPA in the lateral retrochiasmatic area by dopamine (DA) synthesis in the dopaminergic neurons and noradrenaline (NA) synthesis in the noradrenergic afferents of the A15 nucleus. This study also allowed us to test whether DOPAC and HVA molecules released in this medium originate from DA metabolism in dopaminergic cells of the A15 nucleus or in its noradrenergic afferents. We monitored TH activity as well as amine metabolite concentrations in the lateral retrochiasmatic area after a partial destruction of noradrenergic nerves in the brain using intracerebroventricular (i.c.v.) injections of nomifensine followed by 6-hydroxydopamine (6-OHDA) injections. We compared the responses in four experimental ewes with those in four control ewes receiving a single i.c.v. injection of nomifensine only. Decreases in extracellular concentrations of 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG) in the left lateral retrochiasmatic area and the tissue content of NA in the contralateral structure confirmed the effectiveness of the lesion of the noradrenergic endings.(ABSTRACT TRUNCATED AT 400 WORDS)

3,4-Dihydroxyphenylacetic Acid↗
PubMed1994

Evidence that melatonin binding sites in the pars tuberalis do not mediate the photoperiodic actions of melatonin on LH and prolactin secretion in ewes.

An experiment was designed to determine whether the pars tuberalis is the site of action of melatonin involved in the photoperiodic control of LH and prolactin secretion in sheep. In an attempt to produce a 'short-day' effect on these hormones (i.e. stimulation of LH secretion and inhibition of prolactin release), microimplants of melatonin were placed either around the pituitary stalk (n = 6) or in the third ventricle (n = 5) as a control for the efficacy of the microimplant. Two sham-operated groups were treated with empty microimplants around the pituitary stalk (n = 4) or in the third ventricle (n = 3). A further two control groups were given either no melatonin (n = 5) or a melatonin implant s.c. (n = 6). Administration of a melatonin implant s.c. is known to stimulate LH secretion and inhibit prolactin release in photoperiodically inhibited ewes. During the experiment (over 75 days), there was no significant increase in LH concentrations for the ewes receiving melatonin around the pituitary stalk. A similar lack of response was noted in the untreated or sham-treated ewes. In contrast, LH concentrations increased in ewes treated with the melatonin microimplant in the third ventricle on day 37 (+/- 4) and remained high until day 62 (+/- 4). Similarly, in the ewes given an implant s.c., LH concentrations rose on day 39 (+/- 4) and remained high until the end of the study. Furthermore, melatonin caused an inhibition of prolactin secretion relative to controls only when delivered to these sites (i.e. s.c. and into the third ventricle, but not around the pituitary stalk).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗