Efferent projections from the retrochiasmatic area to the median eminence and to the pars nervosa of the hypophysis with special reference to the A15 dopaminergic cell group in the sheep

Regulation of GnRH pulsatility in ewes

Early work in ewes provided a wealth of information on the physiological regulation of pulsatile gonadotropin-releasing hormone (GnRH) secretion by internal and external inputs. Identification of the neural systems involved, however, was limited by the lack of information on neural mechanisms underlying generation of GnRH pulses. Over the last decade, considerable evidence supported the hypothesis that a group of neurons in the arcuate nucleus that contain kisspeptin, neurokinin B and dynorphin (KNDy neurons) are responsible for synchronizing secretion of GnRH during each pulse in ewes. In this review, we describe our current understanding of the neural systems mediating the actions of ovarian steroids and three external inputs on GnRH pulsatility in light of the hypothesis that KNDy neurons play a key role in GnRH pulse generation. In breeding season adults, estradiol (E₂) and progesterone decrease GnRH pulse amplitude and frequency, respectively, by actions on KNDy neurons, with E₂ decreasing kisspeptin and progesterone increasing dynorphin release onto GnRH neurons. In pre-pubertal lambs, E₂ inhibits GnRH pulse frequency by decreasing kisspeptin and increasing dynorphin release, actions that wane as the lamb matures to allow increased pulsatile GnRH secretion at puberty. Less is known about mediators of undernutrition and stress, although some evidence implicates kisspeptin and dynorphin, respectively, in the inhibition of GnRH pulse frequency by these factors. During the anoestrus, inhibitory photoperiod acting via melatonin activates A15 dopaminergic neurons that innervate KNDy neurons; E₂ increases dopamine release from these neurons to inhibit KNDy neurons and suppress the frequency of kisspeptin and GnRH release.

The influence of dopaminergic system inhibition on biosynthesis of gonadotrophin-releasing hormone (GnRH) and GnRH receptor in anoestrous sheep; hierarchical role of kisspeptin and RFamide-related peptide-3 (RFRP-3)

This study aimed to explain how prolonged inhibition of central dopaminergic activity affects the cellular processes governing gonadotrophin-releasing hormone (GnRH) and LH secretion in anoestrous sheep. For this purpose, the study included two experimental approaches: first, we investigated the effect of infusion of sulpiride, a dopaminergic D2 receptor antagonist (D2R), on GnRH and GnRH receptor (GnRHR) biosynthesis in the hypothalamus and on GnRHR in the anterior pituitary using an immunoassay. This analysis was supplemented by analysis of plasma LH levels by radioimmunoassay. Second, we used real-time polymerase chain reaction to analyse the influence of sulpiride on the levels of kisspeptin (Kiss1) mRNA in the preoptic area and ventromedial hypothalamus including arcuate nucleus (VMH/ARC), and RFamide-related peptide-3 (RFRP-3) mRNA in the paraventricular nucleus (PVN) and dorsomedial hypothalamic nucleus. Sulpiride significantly increased plasma LH concentration and the levels of GnRH and GnRHR in the hypothalamic–pituitary unit. The abolition of dopaminergic activity resulted in a significant increase in transcript level of Kiss1 in VMH/ARC and a decrease of RFRP-3 in PVN. The study demonstrates that dopaminergic neurotransmission through D2R is involved in the regulatory pathways of GnRH and GnRHR biosynthesis in the hypothalamic–pituitary unit of anoestrous sheep, conceivably via mechanisms in which Kiss1 and RFRP-3 participate.

Effects of Season and Estradiol on KNDy Neuron Peptides, Colocalization With D2 Dopamine Receptors, and Dopaminergic Inputs in the Ewe

Seasonal reproduction in sheep is primarily due to a dramatic increase in the ability of estradiol (E2) to inhibit the pulsatile secretion of gonadotropin-releasing hormone (GnRH) during the nonbreeding season [anestrus (ANS)]. Recent findings suggest that kisspeptin/neurokinin B/dynorphin (KNDy) neurons of the arcuate nucleus (ARC) play a key role in conveying this negative feedback influence, with dopaminergic projections from the retrochiasmatic area acting upon KNDy cells to decrease kisspeptin release and thus inhibit GnRH pulses. However, several questions remain unanswered: (1) Are the coexpressed KNDy peptides, neurokinin B (NKB) and dynorphin, under seasonal regulation similar to kisspeptin? (2) Are seasonal changes in these peptides and their colocalization of D2 dopamine receptors (D2Rs) steroid dependent? and (3) Do KNDy neurons receive direct input from dopaminergic terminals? We used dual- and triple-label immunofluorescence to analyze brain sections through the ARC of ovariectomized (OVX) and OVX plus E2 ewes perfused during either the breeding season or ANS. Results showed (1) steroid-dependent and steroid-independent seasonal changes in kisspeptin and NKB, but not dynorphin, immunoreactivity; (2) increased D2R coexpression during ANS that was dependent on the presence of E2; and (3) evidence that KNDy cells receive direct contact from dopaminergic terminals and that this input increases during ANS. These results support the hypothesis that dopamine acts to inhibit GnRH secretion in ANS by directly suppressing the activity of ARC KNDy neurons, and implicate NKB as well as kisspeptin in seasonal shifts in E2-negative feedback in the sheep.

Neural mechanisms controlling seasonal reproduction: principles derived from the sheep model and its comparison with hamsters

Seasonal reproduction is a common adaptive strategy among mammals that allows for breeding to occur at times of the year when it is most advantageous for the subsequent survival and growth of offspring. A major mechanism responsible for seasonal reproduction is a striking increase in the responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the negative feedback effects of estradiol. The neural and neuroendocrine circuitry responsible for mammalian seasonal reproduction has been primarily studied in three animal models: the sheep, and two species of hamsters. In this review, we first describe the afferent signals, neural circuitry and transmitters/peptides responsible for seasonal reproductive transitions in sheep, and then compare these mechanisms with those derived from studies in hamsters. The results suggest common principles as well as differences in the role of specific brain nuclei and neuropeptides, including that of kisspeptin cells of the hypothalamic arcuate nucleus, in regulating seasonal reproduction among mammals.

Evidence that dopamine acts via kisspeptin to hold GnRH pulse frequency in check in anestrous ewes

Recent work has implicated stimulatory kisspeptin neurons in the arcuate nucleus (ARC) as important for seasonal changes in reproductive function in sheep, but earlier studies support a role for inhibitory A15 dopaminergic (DA) neurons in the suppression of GnRH (and LH) pulse frequency in the nonbreeding (anestrous) season. Because A15 neurons project to the ARC, we performed three experiments to test the hypothesis that A15 neurons act via ARC kisspeptin neurons to inhibit LH in anestrus: 1) we used dual immunocytochemistry to determine whether these ARC neurons contain D2 dopamine receptor (D2-R), the receptor responsible for inhibition of LH in anestrus; 2) we tested the ability of local administration of sulpiride, a D2-R antagonist, into the ARC to increase LH secretion in anestrus; and 3) we determined whether an antagonist to the kisspeptin receptor could block the increase in LH secretion induced by sulpiride in anestrus. In experiment 1, 40% of this ARC neuronal subpopulation contained D2-R in breeding season ewes, but this increased to approximately 80% in anestrus. In experiment 2, local microinjection of the two highest doses (10 and 50 nmol) of sulpiride into the ARC significantly increased LH pulse frequency to levels 3 times that seen with vehicle injections. Finally, intracerebroventricular infusion of a kisspeptin receptor antagonist completely blocked the increase in LH pulse frequency induced by systemic administration of sulpiride to anestrous ewes. These results support the hypothesis that DA acts to inhibit GnRH (and LH) secretion in anestrus by suppressing the activity of ARC kisspeptin neurons.

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.

Neural systems mediating seasonal breeding in the ewe

Seasonal reproduction in ewes is caused by a dramatic increase in response to oestradiol (E(2)) negative feedback during the nonbreeding (anoestrous) season. Considerable evidence supports the hypothesis that A15 dopaminergic neurones in the retrochiasmatic area (RCh) play a key role in these seasonal changes. These A15 neurones are stimulated by E(2) and inhibit gonadotrophin-releasing hormone (GnRH) secretion in anoestrus, but not the breeding season. Because A15 neurones do not contain oestrogen receptors-alpha (ER alpha), it is likely that E(2)-responsive afferents stimulate their activity when circulating E(2) levels increase during anoestrus. Retrograde tract tracing studies identified a limited set of ER alpha-containing afferents primarily found in four areas [ventromedial preoptic area, RCh, ventromedial and arcuate (ARC) nuclei]. Pharmacological and anatomical data are consistent with GABA- and glutamate-containing afferents controlling A15 activity in anoestrus, with E(2) inhibiting GABA and stimulating glutamate release at this time of year. Tract tracing demonstrated that A15 efferents project posteriorly to the median eminence and the ARC, suggesting possible direct actions on GnRH terminals or indirect actions via kisspeptin neurones in the ARC to inhibit GnRH in anoestrus. Identification of this neural circuitry sets the stage for the development of specific hypotheses for morphological or transmitter/receptor expression changes that would account for seasonal breeding in ewes.

Relationship between polysialylated neural cell adhesion molecule and beta-endorphin- or gonadotropin releasing hormone-containing neurons during activation of the gonadotrope axis in short daylength in the ewe

Morphological plasticity has been demonstrated between breeding and anestrous seasons in the ewe hypothalamus, particularly for the gonadotropin-releasing hormone (GnRH) system. We sought to determine the impact of a photoperiodic transition, from long days (LD, 16 h light/24 h) to short days (SD; 8 h light/24 h), on the association between a marker of cerebral plasticity, the polysialylated form of neural cell adhesion molecule (PSA-NCAM), and two diencephalic populations: the GnRH and beta-endorphin (beta-END) neurons, the latter being potent inhibitors of GnRH neuronal activity. We also estimated the number of contacts on GnRH neurons after the passage to SD, using synaptophysin as a marker for synaptic buttons. Those parameters were evaluated in ovariectomized estradiol-replaced ewes using double immunocytochemistry and confocal microscopy at different times after the transition to SD: day 0 (D0), D30, D45, D60 and D112. Luteinizing hormone (LH) secretion was recorded throughout the experiment. High LH levels were observed only at D112. Significantly more PSA-NCAM was found in the GnRH neuron perimeters in the D112 group than in the other groups. This increase was not associated with any change in the number of synaptophysin-immunoreactive contacts on GnRH neurons. The beta-END peri-neuronal space was affected negatively by the transition to SD: the percentage of PSA-NCAM on beta-END neurons decreased between D45 and D112 in the posterior two thirds of the arcuate nucleus (ARC). These results suggest that photoperiod may reorganize cell interactions in different hypothalamic areas, ultimately reactivating GnRH neurons, in our model of ovariectomized-estradiol replaced ewes.

Axonal projections from the hypothalamus to the pituitary intermediate lobe in rats during ontogenesis: DiI tracing study

This study has determined ontogenetic schedule of axonal arrival from the hypothalamus in the pituitary intermediate lobe (IL) in rats using 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as a retrograde tracer. The brains with attached pituitaries were dissected in rats from the 20th embryonic day (E20) to the 20th postnatal day (P20). Anterior lobe was mechanically detached from the IL, material fixed in paraformaldehyde, and DiI crystals were applied on the IL laying on the posterior lobe (PL). The labeling of IL + PL resulted in staining of hypothalamic magnocellular neurons, which send their axons to the PL, and hypothalamic parvocellular neurons contributing to the innervation of the IL. Therefore, the magnocellular neurons were not taken into account when identifying the neurons projecting axons to the IL. Rare fluorescent neurons projecting their axons to the IL were detected as early as on E20 in the ventral part of the periventricular nucleus (Pe) and in the rostral part of the arcuate nucleus. Few DiI-labeled neurons were seen in Pe from P1 to P3. At P5, the fluorescent neurons were accumulated giving rise to the prominent cluster in the Pe, which was enlarged on later stages and occupied all the Pe. In addition to the Pe, fluorescent neurons first appeared in the retrochiasmatic region and around the ventromedial nucleus in young rats. Thus, the axons of hypothalamic neurons of the Pe and mediobasal hypothalamus first arrive in the IL in rats at the end of intrauterine development, although the principal innervation of the IL is the postnatal event.

Seasonal plasticity within the gonadotropin-releasing hormone (GnRH) system of the ewe: changes in identified GnRH inputs and glial association

The annual reproductive cycle in sheep may reflect a functional remodeling within the GnRH system. Specifically, changes in total synaptic input and association with the polysialylated form of neural cell adhesion molecule have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur or whether glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and nonbreeding season (anestrus) ewes and tested the hypotheses that specific (i.e. gamma-aminobutyric acid, catecholamine, neuropeptide Y, or beta-endorphin) inputs to GnRH neurons change seasonally, and concomitant with any changes in neural inputs is a change in glial apposition. Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirmed that total numbers of neural inputs to GnRH neurons vary with season and demonstrated that specific inputs contribute to these overall changes. Specifically, neuropeptide Y and gamma-aminobutyric acid inputs to GnRH neurons increased during BS and beta-endorphin inputs were greater during either anestrus (GnRH somas) or BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, glial acidic fibrillary protein density, and the thickness of glial fibrils. These findings are interpreted to suggest an increase in net stimulatory inputs to GnRH neurons during the BS contributes to the seasonal changes in GnRH neurosecretion and that this increased innervation is perhaps stabilized by glial processes.

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.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Neuronal projections to the lateral retrochiasmatic area of sheep with special reference to catecholaminergic afferents: immunohistochemical and retrograde tract-tracing studies

The retrochiasmatic area contains the A15 catecholaminergic group and numerous monoaminergic afferents whose discrete cell origins are unknown in sheep. Using tract-tracing methods with a specific retrograde fluorescent tracer, fluorogold, we examined the cells of origin of afferents to the retrochiasmatic area in sheep. The retrogradely labeled cells were seen by observation of the tracer by direct fluorescence or by immunohistochemistry with specific antibodies raised in rabbits or horses. Among the retrogradely labeled neurons, double immunohistochemistry for tyrosine hydroxylase, dopamine-beta-hydroxylase, and serotonin were used to characterize catecholamine and serotonin FG labeled neurons. The retrochiasmatic area, which included the A15 dopaminergic group and the accessory supraoptic nucleus (SON), received major inputs from the lateral septum (LS), the bed nucleus of the stria terminalis (BNST), the thalamic paraventricular nucleus, hypothalamic paraventricular and supraoptic nuclei, the perimamillary area, the amygdala, the ventral part of the hippocampus and the parabrachial nucleus (PBN). Further, numerous scattered retrogradely labeled neurons were observed in the preoptic area, the ventromedial part of the hypothalamus. the periventricular area, the periaqueductal central gray (CG), the ventrolateral medulla and the dorsal vagal complex. Most of the noradrenergic afferents came from the ventro-lateral medulla (Al group), and only a few from the locus coeruleus complex (A6/A7 groups). A few dopaminergic neurons retrogradely labeled with flurogold were observed in the periventricular area of the hypothalamus. Rare serotoninergic fluorogold labeled neurons belonged to the dorsal raphe nucleus. Most of these afferents came from both sides of the brain, except for hypothalamic supraoptic and paraventricular nuclei. In the light of these anatomical data, we compared our results with data obtained from rats, and we discussed the putative role of these afferents in sheep in the regulation of several specific functions in which the retrochiasmatic area may be involved, such as reproduction.

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.

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.