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

Prolactin Mediates Long-Term, Seasonal Rheostatic Regulation of Body Mass in Female Mammals

A series of well-described anabolic and catabolic neuropeptides are known to provide short-term, homeostatic control of energy balance. The mechanisms that govern long-term, rheostatic control of regulated changes in energy balance are less well characterized. Using the robust and repeatable seasonal changes in body mass observed in Siberian hamsters, this report examined the role of prolactin in providing long-term rheostatic control of body mass and photoinduced changes in organ mass (ie, kidney, brown adipose tissue, uterine, and spleen). Endogenous circannual interval timing was observed after 4 months in a short photoperiod, indicated by a significant increase in body mass and prolactin mRNA expression in the pituitary gland. There was an inverse relationship between body mass and the expression of somatostatin (Sst) and cocaine- and amphetamine-regulated transcript (Cart). Pharmacological inhibition of prolactin release (via bromocriptine injection), reduced body mass of animals maintained in long photoperiods to winter-short photoperiod levels and was associated with a significant increase in hypothalamic Cart expression. Administration of ovine prolactin significantly increased body mass 24 hours after a single injection and the effect persisted after 3 consecutive daily injections. The data indicate that prolactin has pleiotropic effects on homeostatic sensors of energy balance (ie, Cart) and physiological effectors (ie, kidney, BAT). We propose that prolactin release from the pituitary gland acts as an output signal of the hypothalamic rheostat controller to regulate adaptive changes in body mass.

Neuronal plasticity and seasonal reproduction in sheep

Seasonal reproduction represents a naturally occurring example of functional plasticity in the adult brain as it reflects changes in neuroendocrine pathways controlling gonadotropin-releasing hormone (GnRH) secretion and, in particular, the responsiveness of GnRH neurons to estradiol negative feedback. Structural plasticity within this neural circuitry may, in part, be responsible for seasonal switches in the negative feedback control of GnRH secretion that underlie annual reproductive transitions. We review evidence for structural changes in the circuitry responsible for seasonal inhibition of GnRH secretion in sheep. These include changes in synaptic inputs onto GnRH neurons, as well as onto dopamine neurons in the A15 cell group, a nucleus that plays a key role in estradiol negative feedback. We also present preliminary data suggesting a role for neurotrophins and neurotrophin receptors as an early mechanistic step in the plasticity that accompanies seasonal reproductive transitions in sheep. Finally, we review recent evidence suggesting that kisspeptin cells of the arcuate nucleus constitute a critical intermediary in the control of seasonal reproduction. Although a majority of the data for a role of neuronal plasticity in seasonal reproduction has come from the sheep model, the players and principles are likely to have relevance for reproduction in a wide variety of vertebrates, including humans, and in both health and disease.

Neuroendocrine control by dopamine of teleost reproduction

While gonadotropin-releasing hormone (GnRH) is considered as the major hypothalamic factor controlling pituitary gonadotrophins in mammals and most other vertebrates, its stimulatory actions may be opposed by the potent inhibitory actions of dopamine (DA) in teleosts. This dual neuroendocrine control of reproduction by GnRH and DA has been demonstrated in various, but not all, adult teleosts, where DA participates in an inhibitory role in the neuroendocrine regulation of the last steps of gametogenesis (final oocyte maturation and ovulation in females and spermiation in males). This has major implications for inducing spawning in aquaculture. In addition, DA may also play an inhibitory role during the early steps of gametogenesis in some teleost species, and thus interact with GnRH in the control of puberty. Various neuroanatomical investigations have shown that DA neurones responsible for the inhibitory control of reproduction originate in a specific nucleus of the preoptic area (NPOav) and project directly to the region of the pituitary where gonadotrophic cells are located. Pharmacological studies showed that the inhibitory effects of DA on pituitary gonadotrophin production are mediated by DA-D2 type receptors. DA-D2 receptors have now been sequenced in several teleosts, and the coexistence of several DA-D2 subtypes has been demonstrated in a few species. Hypophysiotropic DA activity varies with development and reproductive cycle and probably is controlled by environmental cues as well as endogenous signals. Sex steroids have been shown to regulate dopaminergic systems in several teleost species, affecting both DA synthesis and DA-D2 receptor expression. This demonstrates that sex steroid feedbacks target DA hypophysiotropic system, as well as the other components of the brain-pituitary gonadotrophic axis, GnRH and gonadotrophins. Recent studies have revealed that melatonin modulates the activity of DA systems in some teleosts, making the melatonin-DA pathway a prominent relay between environmental cues and control of reproduction. The recruitment of DA neurons for the neuroendocrine control of reproduction provides an additional brain pathway for the integration of various internal and environmental cues. The plasticity of the DA neuroendocrine role observed in teleosts may have contributed to their large diversity of reproductive cycles.

Effects of Prolactin on the Luteinizing Hormone Response to Gonadotropin-Releasing Hormone in Primary Pituitary Cell Cultures During the Ovine Annual Reproductive Cycle

In the sheep pituitary, the localization of prolactin (PRL) receptors in gonadotrophs and the existence of gonadotroph-lactotroph associations have provided morphological evidence for possible direct effects of PRL on gonadotropin secretion. Here, we investigated whether PRL can readily modify the LH response to GnRH throughout the ovine annual reproductive cycle. Cell populations were obtained from sheep pituitaries during the breeding season (BS) and the nonbreeding season (NBS), plated to monolayer cultures for 7 days, and assigned to receive one of the following treatments: 1) nil (control), 2) acute (90- min) bromocriptine (ABr), 3) chronic (7-day) bromocriptine (CBr), 4) ABr and PRL, 5) CBr and PRL, 6) PRL alone, or 7) thyrotropin-releasing hormone. Cells were treated as described above, with the aim of decreasing or increasing the concentrations of PRL in the culture, and simultaneously treated with GnRH for 90 min. The LH concentrations in the medium were then determined by RIA. GnRH stimulated LH in a dose-dependent manner during both stages of the annual reproductive cycle. During the NBS, single treatments did not significantly affect the LH response to GnRH. However, when PRL was combined with bromocriptine, either acutely or chronically, GnRH failed to stimulate LH release at all doses tested (P < 0.01). In contrast, during the BS, the LH response to GnRH was not affected by any of the experimental treatments. These results reveal no apparent effects of PRL alone, but an interaction between PRL and dopamine in the regulation of LH secretion within the pituitary gland, and a seasonal modulation of this mechanism.

Effects of melatonin on luteinizing hormone secretion in anestrous ewes following dopamine and opiate receptor blockade

In the present investigation we have examined the ability of melatonin to modify the pulsatile LH secretion induced by treatment with a DA antagonist (sulpiride, SULP) or opioid antagonist (naloxone, NAL) in intact mid-anestrous ewes. The experimental design comprised the following treatments-in experiment 1: (1) intracerebroventricular (i.c.v.) infusion of vehicle (control I); (2) pretreatment with SULP (0.6 mg/kg subcutaneously) and then i.c.v. infusion of vehicle (SULP + veh); (3) pretreatment with SULP and then i.c.v. infusion of melatonin (SULP + MLT, 100 microg per 100 microl/h, total 400 microg). In experiment 2: (4) i.c.v. infusion of vehicle (control II); (5) i.c.v. infusion of NAL (NAL-alone, 100 microg per 100 microl/h, total 300 microg); (6) i.c.v. infusion of NAL in combination with MLT (NAL + MLT, 100 microg + 100 microg per 100 microl/h). All infusions were performed during the afternoon hours. Pretreatment with SULP induced a significant (P < 0.01) increase in LH pulse frequency, but not in mean LH concentration, compared with control I. In SULP + MLT-treated animals, the LH concentration was significantly (P < 0.01) higher during MLT infusion, but due to highly increased LH secretion in only one ewe. The significant changes in the SULP + MLT group occurred in LH pulse frequency. A few LH pulses were noted after melatonin administration compared with the number during the infusion (P < 0.05) and after vehicle infusion in the SULP + MLT group (P < 0.05). The i.c.v. infusion of NAL evoked a significant increase in the mean LH concentration (P < 0.001) and amplitude of LH pulses (P < 0.01) compared with these before the infusion. The enhanced secretion of LH was also maintained after i.c.v. infusion of NAL (P < 0.01) with a concomitant decrease in LH pulse frequency (P < 0.05). In NAL + MLT-treated ewes, mean plasma LH concentrations increased significantly during and after the infusion compared with that noted before ( P < 0.001). No difference in the amplitude of LH pulses was found in the NAL + MLT group, but this parameter was significantly higher in ewes during infusion of both drugs than during infusion of the vehicle (P < 0.01). The LH pulse frequency differed significantly (p < 0.05), increasing slightly during NAL + MLT administration and decreasing after the infusion. In conclusion, these results demonstrate that: (1) in mid-anestrous ewes EOPs, besides DA, are involved in the inhibition of the GnRH/LH axis; (2) brief administration of melatonin in long-photoperiod-inhibited ewes suppresses LH pulse frequency after the elimination of the inhibitory DA input, but seems to not affect LH release following opiate receptor blockade.

Genistein, a phytoestrogen, effectively modulates luteinizing hormone and prolactin secretion in ovariectomized ewes during seasonal anestrus

Through binding with estrogen receptors, phytoestrogens, plant-derived estrogen-like compounds, affect numerous reproductive functions. It is not known whether these compounds are capable of evoking effective changes in luteinizing hormone (LH) and prolactin (PRL) secretion in ewes by acting directly within the central nervous system (CNS). The hypothesis studied was that genistein, infused for several hours into the third ventricle, could immediately affect LH and PRL secretion in ovariectomized (OVX) ewes during seasonal anestrus. Two doses of genistein, 1 microg/100 microl/h (total 4 microg, n = 7) and 10 microg/100 microl/h (total 40 microg, n = 7), were infused intracerebroventricularly from 12.00 to 16.00 h and blood samples were collected from 8.00 to 20.00 h at 10-min intervals. Randomly selected ewes were infused with a vehicle (control, n = 5). The mean plasma LH concentration in control ewes was significantly (p < 0.01) higher during infusion of the vehicle than before the infusion. It remained on an insignificantly changed level after the infusion. The frequency of LH pulses in control ewes did not differ significantly before, during, or after vehicle infusion. In ewes infused with a lower dose of genistein, plasma LH concentrations decreased significantly (p < 0.001) after the infusion, as compared with the values noted before and during genistein infusion. Only a tendency towards a decrease in LH pulse frequency occurred after infusion of a lower dose of genistein. In ewes infused with a higher dose of genistein, the plasma LH concentration decreased significantly (p < 0.01) after phytoestrogen administration as compared with the values noted before and during infusion. The frequency of LH pulses was also significantly (p < 0.01) lower after genistein administration. Because the changes in PRL secretion were more dynamic in response to genistein infusion, the statistical analysis included 2-hour periods. The mean plasma PRL concentration in control animals was significantly enhanced (p < 0.01) only during the first 2-hour period of sampling. After that it decreased and remained on an unchanged level up to the end of sampling. Similar changes in PRL secretion were observed in both experimental groups before genistein infusion. In contrast, significant (p < 0.01 to p < 0.001) increases in PRL concentration were noted regularly during and shortly after the genistein infusion in either low-dose or high-dose genistein-infused ewes, compared with the concentrations noted before genistein treatment. Plasma PRL concentrations during and after genistein infusion in both experimental groups were also significantly higher than the control (p < 0.01 to p < 0.001). The presented data demonstrate that genistein, a phytoestrogen, may effectively modulate LH and PRL secretion in OVX ewes by acting within the CNS.

Neuroendocrine interactions and seasonality

Sheep in temperate latitudes are seasonal breeders. Of the different seasonal cues, photoperiod is the most reliable parameter and is used by animals as an indication of the time of the year to synchronize endogenous annual rhythms of reproduction and physiology. The photoperiodic information is transduced into neuroendocrine changes through variations in melatonin secretion from the pineal gland. Melatonin triggers variations in the secretion of luteinizing hormone-releasing hormone, luteinizing hormone and follicle stimulating hormone (LHRH/LH/FSH) responsible for seasonal changes in reproductive activity. In female sheep, the seasonal changes in the hormonal LH pattern mainly reflect an increase in the negative feedback exerted by estradiol under long days on the frequency of pulsatile LH secretion. The resulting seasonal inhibition of LH secretion involves the activation of monoaminergic and especially dopaminergic systems by estradiol. Other types of physiological regulation subject to seasonal changes such as voluntary food intake (VFI), fat metabolism, body mass and pelage growth also occur in sheep, goats or related wild species. Several neuroendocrine intermediates seem to be shared by these different systems and may participate in their synchronization, providing the advantage that this helps mammalian species to adapt to their environment.

Noradrenaline and dopamine regulation of prolactin secretion in sheep: role in prolactin homeostasis but not photoperiodism

The role of noradrenaline (NA) and dopamine (DA) in the hypothalamic control of prolactin (PRL) secretion was investigated in hypothalamic intact (control) and hypothalamo-pituitary disconnected (HPD) Soay rams. The animals were exposed to alternating 16-weekly periods of short (8 L : 16D) and long days (16 L : 8D) to induce marked cyclical changes in PRL secretion in both groups (as demonstrated previously). Selective NA and DA receptor antagonists (dose: 1.2 micromol/kg) were administered under short days (low endogenous PRL secretion), and agonists (dose: 0.0012-0.12 micromol/kg) were administered under long days (high endogenous PRL secretion). The acute changes in blood PRL concentrations were measured over 4 h as the index of responsiveness. Under short days, treatment with WB4101 (alpha-1 adenoceptor antagonist), and rauwolscine (alpha-2 antagonist), consistently increased PRL secretion in control, but not in HPD rams. The treatments produced similar acute, drug-specific behavioural effects in both groups. Propranolol (beta antagonist) had no effect on PRL secretion, while sulpiride (DA D-2 antagonist) induced a marked increase in blood PRL concentrations in control rams (> 4 h), and a transient effect in HPD rams (15 min). Under long days, when endogenous PRL secretion was increased, phenylephrine (alpha-1 agonist) produced no effects, while bromocriptine (DA D-2 agonist) robustly decreased PRL concentrations in both control and HPD rams, even at the lowest treatment dose. Overall, the positive responses to the antagonists in the control rams, support the view that DA (acting via D-2 receptors), and to a lesser extent NA (acting via alpha-1/alpha-2 receptors), negatively regulate PRL secretion. In contrast, the lack of responses to the antagonists in the HPD rams, support the view that neither DA, nor NA, mediate the photoperiodic control of PRL secretion.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Melatonin modulation of prolactin and gonadotrophin secretion. Systems ancient and modern

Recent studies in sheep indicate that the pineal melatonin signal which transduces effects of photoperiod acts at separate sites in the pituitary gland and brain to regulate seasonality in prolactin (PRL) and gonadotrophin secretion. The pituitary gland is the proposed site for control of PRL based on the observation that hypothalamo-pituitary disconnected (HPD) rams continue to show normal patterns of PRL secretion in response to changes in photoperiod or treatment with melatonin. Lactotrophs do not express melatonin receptors, thus this pituitary effect is assumed to be mediated by cells in the pars tuberalis via "tuberalin". The mediobasal hypothalamus (MBH) is the putative target for gonadotrophin control since: i) gonadotrophin secretion is dependent on pulsatile GnRH secretion from the MBH, ii) local administration of melatonin in the MBH, but not in other areas of the brain and pituitary gland, readily reactivates GnRH-induced LH and FSH secretion in photo-inhibited rams; and iii) treatment of HPD rams with a chronic pulsatile infusion of GnRH stimulates gonadotrophin secretion irrespective of photoperiod. Complementary studies conducted by others in the Syrian hamster, have shown that lesions in the MBH block the action of melatonin on gonadotrophin but not on prolactin secretion; this supports the "dual-site hypothesis". Since all photoperiodic mammals are essentially similar in hyper-secreting PRL under long days, the pituitary control mechanism for PRL is regarded as conserved (ancient) with the pleiotrophic actions of PRL inducing a summer physiology (e.g. growth of summer pelage). In contrast, the variation between species in the timing of the gonadal cycle indicates that evolution has independently modified the melatonin-sensitive neural circuits in the MBH to permit the species-specific timing of the mating season.

Changes in estrogen receptor expression and cell activity of lactotropes in female mink (Mustela vison) pituitary in response to variations in the gonadal steroid environment

This study was undertaken to get new information on the role played by estrogen (E) on the activity of mink lactotropes. Immunocytochemistry for estrogen receptor (ER) and prolactin (PRL) was applied to assess modifications in the protein production that occur as a result of in vivo changes in the gonadal steroid environment. Variations in the functional activity of lactotropes were demonstrated from the ultrastructural characteristics and morphometric parameters (cellular area, numerical density, and secretory granular size). The present study documents the presence of ER in mink lactotropes revealing the ability of E to regulate the expression of ER in the mink pituitary. Furthermore, all morphological and morphometric parameters of lactotropes activity appeared significantly increased in intact females, killed during the mating period, compared with castrated females under the same photoperiodic conditions. Castration thus blocks the stimulatory effect of photoperiod on metabolic activity of mink lactotropes suggesting that E may participate in the photoperiodic regulation of PRL.

Prolactin replacement fails to inhibit reactivation of gonadotropin secretion in rams treated with melatonin under long days

This study tested the hypothesis that prolactin (PRL) inhibits gonadotropin secretion in rams maintained under long days and that treatment with melatonin (s.c. continuous-release implant; MEL-IMP) reactivates the reproductive axis by suppressing PRL secretion. Adult Soay rams were maintained under long days (16L:8D) and received 1) no further treatment (control, C); 2) MEL-IMP for 16 wk and injections of saline/vehicle for the first 8 wk (M); 3) MEL-IMP for 16 wk and exogenous PRL (s.c. 5 mg ovine PRL 3x daily) for the first 8 wk (M+P). The treatment with melatonin induced a rapid increase in the blood concentrations of FSH and testosterone, rapid growth of the testes, an increase in the frequency of LH pulses, and a decrease in the LH response to N-methyl-D,L-aspartic acid. The concomitant treatment with exogenous PRL had no effect on these reproductive responses but caused a significant delay in the timing of the sexual skin color and growth of the winter pelage. These results do not support the hypothesis and suggest that PRL at physiological long-day concentrations, while being totally ineffective as an inhibitor of gonadotropin secretion, acts in the peripheral tissues and skin to maintain summer characteristics.

Interaction between hypothalamic dopaminergic and opioidergic systems in the photoperiodic regulation of pulsatile luteinizing hormone secretion in sheep

Previous studies in sheep have shown that whereas the inhibitory effects of dopamine (DA) systems on GnRH/gonadotrophin secretion are readily detectable during the sexually inactive phase under long days (LD), the suppressive effects of endogenous opioid peptide (EOP) systems are most evident during the sexually active phase under short days (SD). The hypothesis proposed in this study is that inhibitory DA pathways interact with EOP neurons to regulate GnRH/gonadotropin secretion in sheep and that photoperiod modulates this interaction to relay its effect on the seasonal reproductive cycle. To test this hypothesis, the effects of a DA agonist (bromocriptine) or of a DA antagonist (sulpiride) on the pulsatile LH response to an opioid antagonist (naloxone) were evaluated in sexually active Soay rams exposed to SD, and then reassessed when sexually inactive under LD. The experimental design comprised six treatments: 1) control (vehicle); 2) bromocriptine; 3) sulpiride; 4) naloxone; 5) pretreatment with bromocriptine followed by naloxone; 6) pretreatment with sulpiride followed by naloxone. Under SD, when DA pathways are thought to be quiescent and EOP systems active, bromocriptine suppressed pulsatile LH secretion (P < 0.01), whereas sulpiride had no effect. Under this photoperiod, naloxone induced a conspicuous stimulation of episodic LH release (P < 0.01). This effect was prevented by pretreatment with bromocriptine (P < 0.01), but was not affected by pretreatment with sulpiride. Conversely, under LD, when the activity of DA pathways is thought to be increased and that of EOP systems reduced, bromocriptine was without effect, whereas sulpiride evoked a mild increase in LH pulse frequency (P < 0.05). Under this photoperiod, naloxone induced a smaller stimulation than under SD. This effect was again blocked by pretreatment with bromocriptine but, in contrast to SD, markedly enhanced by pretreatment with sulpiride (P < 0.01). Particularly relevant was that the DA agonist blocked the stimulatory effects of the EOP antagonist under SD, and that the DA antagonist enhanced the effects of the EOP antagonist only under LD. These results are consistent with the hypothesis proposing that, in sheep, DA pathways have a predominant inhibitory effect on both GnRH and EOP neurons, and that changes in day length modulate the interplay between DA and EOP systems as part of the mechanisms involved in the photoperiodic control of the seasonal reproductive cycle.

Median eminence dopaminergic activation is critical for the early long-day inhibition of luteinizing hormone secretion in the ewe

In ewes, photoperiod modulates LH release. The median eminence (ME) dopaminergic activity seems to be implicated in the inhibition of LH secretion by photoperiod. This study investigated the functional importance of ME dopaminergic activity for LH secretion inhibition in three inhibitory photoperiodic treatments: after 33 long days (LD) (LD1 treatment), after 72 LD (LD2 treatment), and after 34 short days. Using reverse microdialysis on three groups of seven ewes, a solution of alpha-methyl-paratyrosine [alphaMPT, an inhibitor of tyrosine hydroxylase (TH); 10 mM in Ringer's lactate] was infused into the ME for 5 h, preceded by a 5-h control period during which only vehicle was infused, in each of the three photoperiodic treatments. AlphaMPT dramatically decreased the 3,4-dihydroxyphenylacetic acid concentration, similarly in all three photoperiodic treatments, suggesting a similar inhibition of TH activity. In the LD1 treatment, alphaMPT significantly increased LH pulse frequency (+1.22 +/- 0.46 pulse/5 h from control period, mean +/- SEM, n = 9; P < 0.05) and mean concentration (+51 +/- 28%; P < 0.001). In the other two photoperiodic treatments, alphaMPT had no significant effect on LH release. Thus, blockade of dopamine synthesis in the ME seems to stimulate LH secretion in early, but not long-term, inhibition by LD nor after the transition to short days. Therefore, dopaminergic activity of the ME seems to be critical for LH secretion inhibition in some photoperiodic inhibitory treatments but not in others.

Pulsatile luteinizing hormone secretion in the ovariectomized, thyroidectomized red deer hind following treatment with dopaminergic and opioidergic agonists and antagonists

Two experiments were conducted to determine whether dopaminergic or opioidergic pathways are modulated by thyroid gland secretions for seasonal suppression of LH secretion in red deer hinds. Ovariectomized (n = 5) or ovariectomized and thyroidectomized (n = 4) hinds, treated with estradiol implants, received the dopamine agonist bromocriptine or the antagonist sulpiride during pulse bleeds in July (breeding season) and October (nonbreeding season). Comparison of July and October mean plasma LH concentration (3.5 +/- 1.3, 0.7 +/- 0.1 ng/ml, respectively), pulse frequency (1.9 +/- 0.4, 0.7 +/- 0.2 pulses/4 h), and pulse amplitude (1.3 +/- 0.5, 0.7 +/- 0. 02 ng/ml) showed lower (p < 0.05) levels in October, and these levels were not significantly affected by thyroidectomy or drug treatment. In the absence of estradiol implants, the hinds received bromocriptine or morphine during the breeding season (July) and their antagonists, sulpiride or naloxone, respectively, in the nonbreeding season (November). In euthyroid hinds there was a seasonal decrease (p < 0.05) in mean plasma LH concentration, pulse frequency, and pulse amplitude, which did not occur in thyroidectomized hinds. There were no effects of drug treatment on LH concentration except for a small increase following sulpiride in November. Plasma prolactin concentration was significantly increased by antagonists and decreased by agonists on most occasions. We conclude that in red deer hinds, seasonal regulation of LH secretion does not involve dopamine or endogenous opioids and the thyroid gland is required specifically for LH suppression in the absence of estradiol.

Blockade of tyrosine hydroxylase activity in the median eminence partially reverses the long day-induced inhibition of pulsatile LH secretion in the ewe

The photoperiod-induced stimulation of LH secretion is associated with a decrease in dopamine content, as well as in the activity of its rate limiting enzyme, tyrosine hydroxylase (TH), in the median eminence (ME) of the ewe. We therefore hypothesize that ME-TH activity can constitute a limiting factor of photoperiod-induced inhibition of LH pulsatile secretion. To test this hypothesis, we studied whether the inhibition of ME-TH activity can reverse the long day-induced inhibition of LH. Using microdialysis, a 3 mM solution of alpha methyl-p-tyrosine (alpha MPT; a competitive inhibitor of TH), was administered in the ME of ovariectomized ewes bearing a 0.5 cm oestradiol implant at the beginning of a LD-induced inhibition of LH secretion. The vehicle solution was infused for 4 h followed by a 3 mM alpha MPT solution infused for an additional 4 h. LH pulsatile secretory patterns within the same animal were compared between the control period and the alpha MPT period. alpha MPT infusion in the ME was associated with an increase in LH pulse frequency whereas it did not affect prolactin secretion. In conclusion, our results suggest that the inhibition of TH activity in the ME causes a stimulation of LH secretion in long-day inhibited ewes.

The rhythmic GABAergic system

GABA is the major inhibitory neurotransmitter in the mammalian brain, and has been implicated in the regulation of a variety of behavioral functions, including biological rhythms. The focus of this minireview is the rhythmic variation of the central GABAergic system, comprising fluctuations of GABA levels and turnover, GABA receptor affinity and postsynaptic activity on the chloride ionophore in rodent's brain. Neurochemical rhythms correlated with diurnal and circadian changes in several behaviors associated with the GABA(A) receptor, e.g., anxiolysis-related behavior. GABA is considered to be the principal neurotransmitter of the mammalian circadian system, being present in the suprachiasmatic nuclei and the intergeniculate leaflet. Pharmacological manipulations of GABA(A) receptors phase shift circadian rhythms and alter circadian responses to light. Administration of putative modulators of GABA function, like melatonin or neuroactive steroids, affects the timing of biological rhythms. Therefore, not only does the GABAergic system exhibit strong diurnal and circadian variations, but it also serves as one of the key modulators of the circadian apparatus.

Effect of oestradiol and photoperiod on TH mRNA concentrations in A15 and A12 dopamine cell groups in the ewe

In the sheep, photoperiod, through melatonin, and oestradiol negative-feedback are two major regulators of seasonal changes in luteinizing hormone (LH) and prolactin secretion. Melatonin and oestradiol act on dopamine neurons of the hypothalamus to modify the enzymatic activity of tyrosine hydroxylase (TH). To further understand how melatonin and oestradiol regulate TH activity, we have studied the level of TH mRNA by in situ hybridization with an homologous cDNA probe, in A12 and A15 dopamine neurons of four groups of ovariectomized ewes: long-day exposed ewes with or without subcutaneous oestradiol implants and short-day exposed ewes with or without oestradiol. Results were analysed in relation to the concentration of LH and prolactin in the peripheral circulation. In the A15 cell group, TH mRNA levels were elevated in the short-day, oestradiol-treated ewes compared to all other groups. In this group, the level of TH mRNA was elevated simultaneously with LH concentration. The low level of TH mRNA found in the long-day, oestradiol-treated ewes may indicate that the increase of TH enzymatic activity previously reported by this treatment is not caused by an increase of the level of enzyme. In the A12 cell group, the level of TH mRNA in both long-day and short-day oestradiol-treated ewes was significantly higher than in ewes without oestradiol replacement. Prolactin concentrations were not correlated with TH mRNA variations in the A12 cell group.

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.

Initial expression of the common alpha-chain in hypophyseal pars tuberalis-specific cells in spontaneous recrudescent hamsters

When exposed to short-day conditions, hamsters and other long-day breeders undergo gonadal regression. With chronic exposure to short days, however, the animals become photorefractory and gonadal recrudescence occurs. The underlying mechanism for this insensitivity is still unknown. There is growing evidence, however, that specific cells of the pituitary pars tuberalis (PT) mediate these photoperiod/nonphotoperiod-dependent changes as a direct or indirect "Zeitgeber" for the endocrine system. We investigated messenger RNA (mRNA)/protein formation for several hypophyseal hormones (beta-TSH, beta-LH, PRL, common alpha-chain) in the pars distalis (PD) and PT of female Djungarian hamsters in long photoperiod (LP) and after 18, 28, and 38 weeks of short photoperiod (SP). As indicated by gonadal and body weight, the hamsters displayed gonadal regression after 18 and 28 weeks of SP; after 38 weeks of SP, all animals showed recrudescence. At 18 and 28 weeks of SP, only PRL mRNA and protein levels were significantly reduced in the PD and returned to LP values after 38 weeks of SP. The expression of hypothalamic tyrosine hydroxylase in the arcuate nucleus that was determined by immunocytochemistry and by in situ hybridization was also down-regulated in SP18 and SP28 with increasing levels at SP38. Urinary 6-sulfatoxymelatonin levels were elevated in SP with highest levels in the SP18 group. In the PT, beta-TSH mRNA and protein were not detectable in all SP groups compared with the moderate signal intensity in LP. The common alpha-chain mRNA and protein, however, which were also reduced in the animals of the SP18 group, were already elevated after 28 weeks of SP and nearly reached LP-levels after 38 weeks of SP. These results show that, in contrast to LH and TSH, PRL expression in the PD is a sensitive indicator for photoperiod dependent changes of the endocrine system and seems to be tyrosine hydroxylase independent. The increase of common alpha-chain expression in PT-specific cells depending upon duration of SP that precedes the hormonal changes in the PD leads us to speculate that PT-specific cells initiate spontaneous recrudescence via a PT-PD pathway.