Identification of dopamine, gonadotrophin-releasing hormone-I, and vasoactive intestinal peptide neurones activated by electrical stimulation to the medial preoptic area of the turkey hypothalamus: a potential reproductive neuroendocrine circuit

Immunolesion of melanopsin neurons causes gonadal regression in Pekin drakes (Anas platyrhynchos domesticus)

Several light sensitive receptors have been described in the avian brain that are thought to regulate the reproductive axis independently from the eyes and pineal gland. Recently, our lab has described the presence of three of these photoneuroendocrine systems in the Pekin duck: opsin, opsin 5, & melanopsin. We set out to test the hypothesis that melanopsin receptive neurons are necessary to maintain seasonal reproductive status along with growth and development in the Pekin drake. To accomplish these goals we first investigated 50-week-old Pekin drakes that were housed in the aviary at Hope College under long day length (18h lights on) conditions in floor pens. To specifically lesion melanopsin-receptive neurons, 3μl of an anti-melanopsin-saporin conjugate (MSAP, 100ng/ul) was injected into the lateral ventricle (n=10). Control drakes were injected with 3μl of equimolar unconjugated anti-melanopsin and saporin (SAP, n=10). Reproductive behaviors were analyzed weekly in a test pen with adult hens and MSAP drakes showed a significant (p<0.01) reduction in reproductive behaviors after week 2. After 5weeks, drakes were euthanized and body weights were measured, and brains, pituitaries, and testes collected and stored for analyses. Mature MSAP-treated drakes had significantly (p<0.001) reduced relative teste weights compared to SAP controls. qRT-PCR analyses of hypothalamus showed a significant reduction (p<0.001) in GnRH and melanopsin mRNA levels, but not opsin 5, vertebrate ancient opsin, or opsin 2 (rhodopsin). Immunocytochemical analyses showed a significant reduction (p<0.01) in tyrosine hydroxylase-immunoreactivity in the PMM. These data suggest that although blue light alone is not able to maintain testicular function, the blue-light sensitive melanopsin activity is critical to maintain gonadal function.

Dopamine and prolactin involvement in the maternal care of chicks in the native Thai hen (Gallus domesticus)

The dopaminergic (DAergic) system plays a pivotal role in incubation behavior via the regulation of prolactin (PRL) secretion in birds, however the role of the DA/PRL system in rearing behavior is poorly understood. The objective of this study was to investigate the relationship between the DA/PRL system and rearing behavior in a gallinaceous bird, the native Thai chicken. Incubating native Thai hens were divided into two groups. In the first group, hens were allowed to care for their chicks (rearing hens; R). In the second group, hens were deprived of their chicks immediately after hatching (non-rearing hens; NR). In both groups, blood samples and brain sections were collected at different time points after the chicks hatched (days 4, 7, 10, 14, 17, 21, 24, and 28; 6 hens/time point/group). In this study, tyrosine hydroxylase (TH) was used as a marker for DAergic neurons. The numbers of TH-immunoreactive (-ir) neurons in the nucleus intramedialis (nI) and in the nucleus mamillaris lateralis (ML), which regulate the vasoactive intestinal peptide (VIP)/PRL system, were determined in R and NR hens utilizing immunohistochemical techniques. Plasma PRL levels were determined by enzyme-linked immunosorbent assays. The results revealed that both the number of TH-ir neurons in the nI and the plasma PRL levels were significantly higher in the R hens compared with the NR hens during the first 14 days of chick rearing (P<0.05). However, there was no significant change in the DAergic activity in the ML in either the R or NR groups throughout the 28-day rearing periods. These results suggest that the DA/PRL system is involved in early rearing behavior. The additional decline in DAergic activity and plasma PRL levels during the disruption of rearing behavior further supports their involvement in rearing behavior in this equatorial precocial species.

Functional interactions of dopamine cell groups reflect personality, sex, and social context in highly social finches

Dopamine (DA) is well known for its involvement in novelty-seeking, learning, and goal-oriented behaviors such as social behavior. However, little is known about how DA modulates social processes differentially in relation to sex and behavioral phenotype (e.g., personality). Importantly, the major DA cell groups (A8-A15) are conserved across all amniote vertebrates, and thus broadly relevant insights may be obtained through investigations of avian species such as zebra finches (Taeniopygia guttata), which express a human-like social organization based on biparental nuclear families that are embedded within larger social groups. We here build upon a previous study that quantified multidimensional personality structures in male and female zebra finches using principal components analysis (PCA) of extensive behavioral measures in social and nonsocial contexts. These complex dimensions of behavioral phenotype can be characterized as Social competence/dominance, Gregariousness, and Anxiety. Here we analyze Fos protein expression in DA neuronal populations in response to social novelty and demonstrate that the Fos content of multiple dopamine cell groups is significantly predicted by sex, personality, social context, and their interactions. In order to further investigate coordinated neuromodulation of behavior across multiple DA cell groups, we also conducted a PCA of neural variables (DA cell numbers and their phasic Fos responses) and show that behavioral PCs are associated with unique suites of neural PCs. These findings demonstrate that personality and sex are reflected in DA neuron activity and coordinated patterns of neuromodulation arising from multiple DA cell groups.

Differential roles of hypothalamic serotonin receptor subtypes in the regulation of prolactin secretion in the turkey hen

In the turkey, exogenous serotonin (5-hydroxytryptamine, 5-HT) increases prolactin (PRL) secretion by acting through the dopaminergic (DAergic) system. In the present study, infusion of the 5-HT(2C) receptor agonist, (R)(-)-DOI hydrochloride (DOI), into the third ventricle stimulates PRL secretion, whereas the 5-HT(1A) receptor agonist, (+/-)-8-OH-DPAT hydrobromide (DPAT), inhibits PRL secretion. Using the immediate-early gene, c-fos, as an indicator of neuronal activity, in situ hybridization histochemistry showed preferential c-fos co-localization within tyrosine hydroxylase immunoreactive neurons (the rate limiting enzyme in DA synthesis) in the areas of the nucleus preopticus medialis (POM) and the nucleus premammillaris (PMM), in response to DPAT and DOI, respectively. To clarify the involvement of 5-HT(1A) and 5-HT(2C) receptors in PRL regulation, their mRNA expression was determined on hypothalamic tissue sections from birds in different reproductive stages. A significant difference in 5-HT1A receptor was observed, with the POM of hypoprolactinemic short day and photorefractory birds showing the highest expression. 5-HT2C receptors mRNA did not change during the reproductive cycle. The data presented support the notion that DA neurons in the PMM and POM mediate the stimulatory and inhibitory effects of 5-HT, respectively, on PRL secretion and the 5-HTergic system can both stimulate and inhibit PRL secretion.

Gonadotropin releasing hormone and brooding behavior in the native Thai hen

Changes in the number of hypothalamic gonadotropin releasing hormone-I (GnRH-I) neurons within the Nucleus commissurae pallii (nCPa) were associated with the reproductive cycle of native Thai chickens. In order to further understand the association of GnRH-I in the regulation of brooding behavior in this bird, the native Thai chickens were divided into two groups; chick-rearing (R) and non-chick-rearing (NR) hens. Numbers of visible of GnRH-I-immunoreactive (GnRH-I-ir) neurons in the hypothalamus of R and NR hens were compared utilizing immunohistochemistry. Numbers of visible GnRH-I-ir neurons within the Nucleus anterior medialis hypothalami, Nucleus suprachaiasmaticus, pars medialis, Nucleus septalis lateralis, Nucleus paraventricularis magnocellularis, and Regio lateralis hypothalami areas were observed in both groups, but no differences were seen between R and NR hens. The number of visible GnRH-I neurons in the nCPa was higher (P<0.05) in the NR than in R hens, and increased in NR hens by day 14 after chick removal. These findings suggest, for the first time, an association of the GnRH system with brooding behavior in continuously breeding birds. Furthermore, the expression of brooding behavior of native Thai chickens might be regulated, in part, by GnRH-I neurons in the nCPa.

Distribution and variation in gonadotropin releasing hormone-I (GnRH-I) immunoreactive neurons in the brain of the native Thai chicken during the reproductive cycle

Gonadotropin releasing hormone-I (GnRH-I) is known to regulate the avian reproductive system. We investigated the roles of GnRH-I in the regulation of the reproductive system of the native Thai chicken. The distribution of GnRH-I neurons and changes in GnRH-I-immunoreactive (-ir) neurons throughout the reproductive stages and between incubating and nest-deprived hens were analyzed utilizing immunohistochemical techniques. The results revealed that GnRH-I-ir neurons were distributed in a discrete region lying close to the third ventricle from the level of preoptic area through the anterior hypothalamus, with the greatest abundance found within the nucleus commissurae pallii (nCPa). The number of GnRH-I-ir neurons in the nCPa was highest in laying hens when compared with that in the other reproductive stages. Nest deprivation caused an increase in the number of GnRH-I-ir neurons in the nCPa of nest-deprived hens when compared with incubating hens. These results indicate that GnRH-I expression is correlated with the reproductive state in the native Thai chicken and may be, in part, regulated by it. This study also confirms a pivotal role of GnRH-I in controlling avian reproduction of this non-seasonal breeding, equatorial species.

Dopamine and mesotocin neurotransmission during the transition from incubation to brooding in the turkey

We investigated the neuroendocrine changes involved in the transition from incubating eggs to brooding of the young in turkeys. Numbers of mesotocin (MT; the avian analog of mammalian oxytocin) immunoreactive (ir) neurons were higher in the nucleus paraventricularis magnocellularis (PVN) and nucleus supraopticus, pars ventralis (SOv) of late stage incubating hens compared to the layers. When incubating and laying hens were presented with poults, all incubating hens displayed brooding behavior. c-fos mRNA expression was found in several brain areas in brooding hens. The majority of c-fos mRNA expression by MT-ir neurons was observed in the PVN and SOv while the majority of c-fos mRNA expression in dopaminergic (DAergic) neurons was observed in the ventral part of the nucleus preopticus medialis (POM). Following intracerebroventricular injection of DA or oxytocin (OT) receptor antagonists, hens incubating eggs were introduced to poults. Over 80% of those injected with vehicle or the D1 DA receptor antagonist brooded poults, while over 80% of those receiving the D2 DA receptor antagonist or the OT receptor antagonist failed to brood the poults. The D2 DA/OT antagonist groups also displayed less c-fos mRNA in the dorsal part of POM and the medial part of the bed nucleus of the stria terminalis (BSTM) areas than did the D1 DA/vehicle groups. These data indicate that numerous brain areas are activated when incubating hens initially transition to poult brooding behavior. They also indicate that DAergic, through its D2 receptor, and MTergic systems may play a role in regulating brooding behaviors in birds.

Genetic effects of polymorphisms in candidate genes and the QTL region on chicken age at first egg

Background

The age at first egg (AFE), an important indicator for sexual maturation in female chickens, is controlled by polygenes. Based on our knowledge of reproductive physiology, 6 genes including gonadotrophin releasing hormone-I (GnRH-I), neuropeptide Y (NPY), dopamine D2 receptor (DRD2), vasoactive intestinal polypeptide (VIP), VIP receptor-1 (VIPR-1), and prolactin (PRL), were selected as candidates for influencing AFE. Additionally, the region between ADL0201 and MCW0241 of chromosome Z was chosen as the candidate QTL region according to some QTL databases. The objective of the present study was to investigate the effects of mutations in candidate genes and the QTL region on chicken AFE.

Results

Marker-trait association analysis of 8 mutations in those 6 genes in a Chinese native population found a highly significant association (P < 0.01) between G840327C of the GnRH-I gene with AFE, and it remained significant even with Bonferroni correction. Based on the results of the 2-tailed χ² test, mutations T32742394C, T32742468C, G32742603A, and C33379782T in the candidate QTL region of chromosome Z were selected for marker-trait association analysis. The haplotypes of T32742394C and T32742468C were significantly associated (P < 0.05) with AFE. Bioinformatics analysis indicated that T32742394C and T32742468C were located in the intron region of the SH3-domain GRB2-like 2 (SH3GL2) gene, which appeared to be associated in the endocytosis and development of the oocyte.

Conclusion

This study found that G840327C of the GnRH-I gene and the haplotypes of T32742394C-T32742468C of the SH3GL2 gene were associated with the chicken AFE.

Changes in vasoactive intestinal peptide and tyrosine hydroxylase immunoreactivity in the brain of nest-deprived native Thai hen

Hyperprolactinemia is associated with incubation behavior and ovarian regression in birds. To investigate the association of prolactin (PRL), vasoactive intestinal peptide (VIP), and dopamine (DA) with the neuroendocrine regulation of incubation behavior, changes in the number of visible VIP-immunoreactive (VIP-ir) neurons in the nucleus inferioris hypothalami (IH) and nucleus infundibuli hypothalami (IN) and tyrosine hydroxylase-immunoreactive (TH-ir) neurons in the nucleus intramedialis (nI) and nucleus mamillaris lateralis (ML) of incubating native Thai hens were compared with those of nest-deprived hens. TH was used as a marker for dopaminergic (DAergic) neurons. Blood samples were collected to determine PRL levels. The localization and the number of visible VIP-ir and TH-ir neurons were determined by immunohistochemistry. Disruption of incubation behavior was accompanied by a precipitous decline in plasma PRL levels. The number of visible VIP-ir neurons in the IH-IN and TH-ir neurons in the nI and ML were high during incubation and decreased when hens were deprived of their nests. This study indicated an association between VIP neurons in the IH-IN and DA neurons in the nI and ML with the degree of hyperprolactinemia, suggesting that the expression of incubation behavior in birds might be, in part, regulated by the DAergic input from the nI and ML to VIP neurons in the IH-IN and subsequent PRL release.

Activity of hypothalamic dopaminergic neurones during the day of oestrus: involvement in prolactin secretion

A secretory surge of prolactin occurs on the afternoon of oestrus in cycling rats. Pituitary prolactin is inhibited by dopamine. We evaluated the activity of the neuroendocrine dopaminergic neurones during oestrus and dioestrus, as determined by dopaminergic activity in the median eminence and neurointermediate lobe of the pituitary, as well as Fos-related antigen expression in tyrosine hydroxylase (TH)-immunoreactive (ir) neurones of the arcuate nucleus (ARC) and periventricular nucleus (Pe). During oestrus, the 4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence decreased at 16.00 h, coinciding with the increase in plasma prolactin levels. Similarly, the expression of Fos-related antigen in TH-ir neurones of Pe and rostral-, dorsomedial- and caudal-ARC also decreased at 16.00 h. On dioestrus, 4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence and Fos-related antigen expression in TH-ir neurones of Pe and rostral-ARC decreased at 18.00 h, whereas prolactin levels were unaltered. No variation in dopaminergic activity was found in the neurointermediate lobe of the pituitary on either oestrus or dioestrus. The number of TH-ir neurones in the ARC and parameters of dopaminergic activity were found to be generally lower on oestrus compared to dioestrus. The transitory decrease in the activity of neuroendocrine dopaminergic neurones temporally associated with the prolactin surge on the afternoon of oestrus suggests a role for dopamine in the generation of the oestrous prolactin surge.

Gonadotrophin-releasing hormone neurons in a photoperiodic songbird express fos and egr-1 protein after a single long day

Birds use a variety of environmental cues, such as day length, temperature and social interactions, to time reproductive efforts. For most seasonally breeding birds, day length is the most important cue and takes precedence over all others. Experimental manipulation of day length has shown that, in a number of galliformes and passeriformes, exposure to a single long day induces a rise in plasma luteinising hormone (LH). The mechanisms underlying this response are only beginning to be understood. In Japanese quail and Zonotrichia sparrows, one long day causes striking up-regulation of the protein products of immediate early genes (IEGs) in the mediobasal hypothalamus, near gonadotrophin-releasing hormone (GnRH) axons and terminals. Photoperiodic induction of the same proteins in the GnRH somata themselves, however, has not been described in these species. In the present study, we used immunohistochemistry to assay the induction of two IEGs, Fos and Egr-1, in the GnRH somata of male and female white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. We found that immunoreactivity for both proteins increased in a subset of the GnRH neurones of the septo-preoptic area by the morning after the long day. Photo-induced expression of Egr-1 or Fos protein in GnRH neurones was limited to a population of cells in the medial preoptic area. Males showed significantly greater induction of both proteins in this population of GnRH neurones than did females, which is consistent with the hypothesis that males may be more sensitive to photic cues. Overall, the results obtained suggest that photostimulation stimulates new protein synthesis in GnRH neurones on a relatively rapid time scale. Further research is required to determine whether the GnRH somata are themselves integrating photic cues, or whether they are responding rapidly to an increased demand for GnRH synthesis.

Serotonin receptor subtypes influence prolactin secretion in the turkey

Serotonin (5-HT) stimulation of prolactin (PRL) secretion is mediated through the dopaminergic (DAergic) system, with 5-HT ligands having no direct effect on pituitary PRL release. Infusion of 5-HT into the third ventricle (ICV) or electrical stimulation (ES) of the medial preoptic area (POM) or the ventromedial nucleus (VMN) induces an increase in circulating PRL in the turkey. These increases in PRL do not occur when a selective antagonist blocks the D(1) dopamine (DA) receptors in the infundibular area (INF). In this study, the ICV infusion of (R)(-)-DOI hydrochloride (DOI), a selective 5-HT(2A) eceptor agonist, caused PRL to increase. Pretreatment with Ketanserin tartrate salt (KETAN), a selective 5-HT(2A) receptor antagonist, blocked DOI-induced PRL secretion, attesting to the specificity of the response. DOI-induced PRL secretion was prevented when the D(1) DA receptors in the INF were blocked by the D(1) DA receptor antagonist, R(+)-SCH-23390 hydrochloride microinjection, suggesting that the DAergic activation of the vasoactive intestinal peptide (VIP)/PRL system is mediated by a stimulatory 5-HT(2A) receptor subtype. The DOI-induced PRL increase did not occur when (+/-)-8-OH-DPAT (DPAT) was concurrently infused. DPAT is a 5-T(1A) receptor agonist which appears to mediate the inhibitory influence of 5-HT on PRL secretion. When DPAT was microinjected directly into the VMN, it blocked the PRL release affected by ES in the POM. These data suggested that when 5-HT(2A) receptors are activated, they influence the release of DA to the INF. When 5-HT(1A) receptors are stimulated, they somehow inhibit the PRL-releasing actions of 5-HT(2A) receptors. This inhibition could take place centrally, or it could occur postsynaptically at the pituitary level. It is known that D(2) DA receptors in the pituitary antagonize PRL-releasing effect of VIP. A release of DA to the pituitary, initiated by 5-HT(1A) receptors, could effectively inhibit PRL secretion.

Serotonergic and catecholaminergic interactions with co-localised dopamine-melatonin neurones in the hypothalamus of the female turkey

Serotonin and catecholamines (dopamine, norepinephrine, epinephrine) have important roles as neurotransmitters in avian reproduction, but their anatomical relationship to the neuroendocrine circuitry that regulates reproduction is poorly understood. Our previous studies have shown that co-localised dopamine-melatonin (DA-MEL) neurones in the avian premammillary nucleus (PMM) are active during periods of photoresponsiveness and, therefore, are potentially photosensitive neurones. Because serotonergic and catecholaminergic neurotransmitters are important regulators of reproductive function in the female turkey, we hypothesised that the serotonergic/catecholaminergic neurones within the brainstem might interact with PMM DA-MEL neurones and constitute an important circuit for reproductive function. To examine this possible interaction, the retrograde fluorescent tract tracer, 1,1'dioctadecyl-3,3,3'3'-tetramethyleindocarbocyanine perchlorate (DiI) was injected into the PMM, and combined with serotonin, tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH) and phenyl N-methyltransferse (PNMT) immunocytochemistry to reveal neuroanatomical connections. Changes in the activities of serotonergic, dopaminergic, adrenergic and noradrenergic neuronal systems projecting to the PMM were measured at different reproductive states with in situ hybridisation (ISH) techniques, using tryptophan hydroxylase 2 (TPH2) and TH mRNA expression, respectively. Cells labelled with DiI were found in anatomically discrete areas in or near the hypothalamus and the brainstem. Double immunocytochemistry confirmed that there were serotonin, DBH and PNMT fibres in close apposition to DA-MEL neurones. TPH2 mRNA expression in serotonin neurones was found in several nuclei, and its most abundant mRNA expression was seen in the nucleus Locus ceruleus of laying and incubating hens. TH mRNA expression levels in the six catecholaminegic areas labelled with DiI was measured across the different reproductive states. In the nucleus tractus solitarius (adrenergic), the highest level of TH mRNA expression was found in photorefractory hens and the lowest level in incubating hens. These observed patterns of serotonin/catecholamine neuronal distribution and their variable interactions with PMM DA-MEL neurones during different reproductive states may offer a significant neuroanatomical basis for understanding the control of avian reproductive seasonality.

Vasoactive intestinal peptide and its role in continuous and seasonal reproduction in birds

Native Thai chicken, an equatorial species breeds throughout the year, whereas turkeys are seasonal temperate zone breeder whose reproductive cycle is terminated by the onset of photorefractoriness. This study investigated VIPergic activity throughout a reproductive cycle in both species, hypothesizing that the differential expression of vasoactive intestinal peptide (VIP) would provide an insight into the differing reproductive strategies of the two species. Distribution of VIP neurons in the native Thai chicken and a comparison of VIPergic activity in the nucleus inferioris hypothalami (IH) and nucleus infundibuli hypothalami (IN) were investigated. VIP immunoreactivity was found throughout the native Thai chicken brain, predominantly located within the IH-IN. The pattern of VIP distribution in the native Thai chicken supports the findings reported in temperate zone species. Unlike the turkey, where there is a dissociation between VIPergic activity and prolactin levels during photorefractoriness, in the native Thai chicken, which do not express photorefractoriness, changes in VIP immunoreactive (VIP-ir) neurons within the IH-IN were directly correlated with prolactin throughout the reproductive cycle. VIPergic activity reached its lowest level after hatching of the chicks in the native Thai chicken, while in the turkey VIPergic activity was lowest only after exposure to a short day photoperiod and the acquisition of photosensitivity. This suggests that VIP neurons in the IH-IN may play a pivotal role in regulating the reproductive cycle and its differential expression following hatching of the young may, in part, account for the difference in reproductive mode between equatorial, continually breeding, non-photoperiodic birds and seasonally breeding, photoperiodic birds.

The dopaminergic system in the brain of the native Thai chicken, Gallus domesticus: localization and differential expression across the reproductive cycle

Dopamine (DA) has a pivotal role in avian prolactin (PRL) secretion, acting centrally through D(1) DA receptors to stimulate PRL secretion by operating through vasoactive intestinal peptide (VIP). DA also inhibits PRL secretion by activating D(2) DA receptors at the pituitary level. This study was designed to investigate the distribution of DA neurons in the native Thai chicken, utilizing tyrosine hydroxylase (TH) as a marker for dopaminergic neurons. The differential expression of hypothalamic TH immunoreactive (TH-ir) neurons was also compared across the reproductive cycle. The results revealed that TH-ir neurons and fibers were found throughout the brain of the laying hen and were predominantly located within the diencephalon and mesencephalon. The observed distribution pattern of TH immunoreactivity was consistent with that reported previously in several avian species. However, changes in the number of TH-ir neurons in the nucleus intramedialis (nI) were observed across the reproductive cycle and correlated directly with variations in PRL levels. The population of TH-ir neurons in the nI increased significantly during the egg incubation period, where circulating PRL levels were the greatest. This study indicates, for the first time, that an association exists between DA neurons and the regulation of the reproductive system in the native Thai chicken. There is a paucity of information about the reproductive neuroendocrine regulation of tropical non-seasonally breeding avian species and it is suggested that the differential expression of DA neurons in the nI might play a role in the control of VIP secretion and subsequent PRL release in such birds.

Alpha-oestrogen and progestin receptor expression in the hypothalamus and preoptic area dopaminergic neurones during oestrous in cycling rats

A secretory surge of prolactin occurs on the afternoon of oestrous in cycling rats. Although prolactin is regulated by ovarian steroids, plasma oestradiol and progesterone levels do not vary during oestrous. Because prolactin release is tonically inhibited by hypothalamic dopamine and modulated by dopamine transmission in the preoptic area (POA), the present study aimed to evaluate whether oestrogen receptor (ER)-alpha and progestin receptor (PR) expression in the dopaminergic neurones of arcuate (ARC), periventricular, anteroventral periventricular (AVPe) and ventromedial preoptic (VMPO) nuclei changes during the day of oestrous. Cycling rats were perfused every 2 h from 10-20 h on oestrous. Brain sections were double-labelled to ERalpha or PR and tyrosine hydroxylase (TH). The number of TH-immunoreactive (ir) neurones did not vary significantly in any area evaluated. ERalpha expression in TH-ir neurones increased at 14 and 16 h in the rostral-ARC and dorsomedial-ARC, 14 h in the caudal-ARC and 16 h in the VMPO, whereas it was unaltered in the ventrolateral-ARC, periventricular and AVPe. PR expression in TH-ir neurones of the periventricular and rostral, dorsomedial, ventrolateral and caudal-ARC decreased transitorily during the afternoon, showing the lowest levels between 14 and 16 h; but it did not vary in the AVPe and VMPO. Plasma oestradiol and progesterone concentrations were low and unaltered during oestrous, indicating that the changes in receptors expression were probably not due to variation in ligand levels. Thus, our data suggest that variations in ERalpha and PR expression may promote changes in the activity of medial basal hypothalamus and POA dopaminergic neurones, even under unaltered secretion of ovarian steroids, which could facilitate the occurrence and modulate the magnitude of the prolactin surge on oestrous.

Dopamine-melatonin neurons in the avian hypothalamus controlling seasonal reproduction

Day length cues are used by temperate zone birds to time seasonal changes in reproductive physiology and behavior. However, the neuronal and neurochemical circuits used to measure day length (photoperiodic time measurement; PTM), transduce light information and activate the reproductive neuroendocrine system have not been definitely established. Recent findings from our laboratory provide data showing dopamine (DA) neurons within the premammillary nucleus (PMM) of the caudal turkey hypothalamus are putative photoreceptive neurons. These neurons reach threshold activation when a brief pulse of light is provided during the photo-inducible phase for photosexual stimulation. To further clarify the role of PMM neurons in coding daylight information, we showed that by using double-label immunocytochemistry (ICC) these neurons are immunoreactive (ir) to both tyrosine hydroxylase (TH; the rate limiting enzyme in DA biosynthesis) and melatonin (MEL). Moreover, we found these neurons to express tryptophan hydroxylase 1 (TPH1; the first enzyme in MEL biosynthesis) and 5-HT N-acetyltransferase (AANAT; a key regulatory enzyme in MEL synthesis) mRNAs but not neuronal tryptophan hydroxylase 2 mRNA (TPH 2; the rate limiting enzyme in 5-HT pathway). Both TH and TPH1 mRNAs were shown to cycle rhythmically, and with opposite phases, in PMM neurons of birds kept under a diurnal illumination cycle (12-h light/dark; LD). These neurons could also generate 24 h TH and TPH1 mRNA expression rhythms with the same phase relationship in constant light (LL) and constant dark (DD). In addition, the expression patterns and amplitudes of TH and TPH1 mRNAs were different between long and short photoperiods. These findings may form the basis for an endogenous dual-oscillator circadian system within PMM DA-MEL co-localized neurons controlling reproductive seasonality in birds.

Rhythmic dependent light induction of gonadotrophin-releasing hormone-I expression and activation of dopaminergic neurones within the premammillary nucleus of the turkey hypothalamus

Our previous studies using turkey hens have demonstrated that c-fos mRNA (a marker of neuronal activation) is expressed in gonadotrophin-releasing hormone-I (GnRH-I), vasoactive intestinal peptide (VIP) and dopamine (DA) neurones following electrical stimulation in the preoptic area. DA has been shown to have both stimulatory and inhibitory effects on the GnRH-I/luteinising hormone (LH), follicle-stimulating hormone (FSH) and VIP/prolactin (PRL) systems. To identify the DA neurones that mediate the stimulatory influences of photoperiod on the reproductive system, we examined c-fos mRNA induction in DA, GnRH-I, and VIP neurones in the turkey hypothalamus using a dark-interruption experimental design. A 30-min light period was provided to short day (6L : 18D) photosensitive turkeys at times when birds were responsive to light (14 h after first light) and at times when birds were unresponsive to light (8 h and 20 h after first light). The only area where DA neurones were activated when the birds were provided with light was in the nucleus premammillaris (PMM). The number of activated DA neurones was significantly greater when light was provided at 14 h (during the photoinducible phase) than at 8 h or 20 h. At 14 h, there was also an increase in the number of GnRH-I neurones activated in the area of the nucleus commissura pallii (nCPa), as well as an up-regulation of GnRH-I mRNA expression. No expression of c-fos mRNA was observed in VIP neurones in the nucleus infundibularis or up-regulation of VIP mRNA expression in any of the experimental light treatments. These results are the first evidence to demonstrate a relationship between the dopaminergic system in the PMM and the GnRH-I system in the nCPa during the photoinduction of avian reproductive activity.

Prolactin secretory surge during estrus coincides with increased dopamine activity in the hypothalamus and preoptic area and is not altered by ovariectomy on proestrus

Prolactin (PRL) secretory surges have been reported on the afternoons of both proestrus and estrous in cycling rats. As neuroendocrine regulation of estrous PRL surge is poorly understood, the present study aimed to investigate the involvement of hypothalamic dopamine and serotonin as well as of plasma ovarian steroids in this hormonal surge generation. For that, we determined the concentrations of dopamine, serotonin and their respective metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole-3-acetic acid (5-HIAA) in the mediobasal hypothalamus (MBH) and medial preoptic area (MPOA) throughout the day of estrus and correlated them with plasma PRL levels. In a second study, we evaluated the effect of ovariectomy on the morning of proestrus on PRL surges of both proestrus and estrus. Dopamine turnover, as determined by DOPAC/dopamine ratio, increased in both the MBH and MPOA coinciding with the afternoon PRL surge on estrus. In contrast, both the concentration and turnover (5-HIAA/serotonin) of serotonin within these areas were unaltered during estrus. In addition, ovariectomy reduced plasma estradiol and progesterone levels but did not alter the PRL surges on proestrus and estrus. Considering that dopamine is the main inhibitor of PRL release and that PRL auto-regulates its secretion through a short-loop feedback mechanism, our present results suggest that PRL may suppress its own secretion during the estrus surge through the activation of the dopaminergic neurons in the MBH and MPOA. In addition, the PRL surge on estrus seems do not depend on either the activity of hypothalamic serotonin or the increased secretion of ovarian steroids on proestrus.

Rhythm-dependent light induction of the c-fos gene in the turkey hypothalamus

Day length (photoperiod) is a powerful synchroniser of seasonal changes in the reproductive neuroendocrine activity in temperate-zone birds. When exposed to light during the photoinducible phase, reproductive neuroendocrine responses occur. However, the neuroendocrine systems involved in avian reproduction are poorly understood. We investigated the effect of light exposure at different circadian times upon the hypothalamus and components of the circadian system, using c-fos mRNA expression, measured by in situ hybridisation, as an indicator of light-induced neuronal activity. Levels of c-fos mRNA in these areas were compared after turkey hens (on a daily 6-h light period) had been exposed to a 30-min period of light occurring at 8, 14, or 20 h after the onset of first light of the day (subjective dawn). Non-photostimulated control birds were harvested at the same times. In birds, photostimulated within the photoinducibile phase (14 h), in contrast to before or after, c-fos mRNA was significantly increased in the nucleus commissurae pallii (nCPa), nucleus premamillaris (PMM), eminentia mediana (ME), and organum vasculosum lamina terminalis (OVLT). Photostimulation increased c-fos mRNA expression in the pineal gland, nucleus suprachiasmaticus, pars visualis (vSCN) and nucleus inferioris hypothalami compared to that of their corresponding nonphotostimulated controls. However, the magnitudes of the responses in these areas were similar irrespective of where in the dark period the pulses occurred. No c-fos mRNA was induced in the nucleus infundibulari, in response to the 30-min light period at any of the circadian times tested. The lack of c-fos up-regulation in the pineal gland and vSCN following photostimulation during the photoinducible phase lends credence to the hypothesis that these areas are not involved in the photic initiation of avian reproduction. On the other hand, c-fos mRNA increases in the nCPa, ME, and OVLT support other studies showing that these areas are involved in the onset of reproductive behaviour initiated by long day lengths. The present study provides novel data showing that the PMM in the caudal hypothalamus is involved in the neuronally mediated, light-induced initiation of reproductive activity in the turkey hen.