Distribution of neuropeptides in the infundibular nucleus of the sheep

KNDy neurone activation prior to the LH surge of the ewe is disrupted by LPS

In the ewe, steroid hormones act on the hypothalamic arcuate nucleus (ARC) to initiate the GnRH/LH surge. Within the ARC, steroid signal transduction may be mediated by estrogen receptive dopamine-, β-endorphin- or neuropeptide Y (NPY)-expressing cells, as well as those co-localising kisspeptin, neurokinin B (NKB) and dynorphin (termed KNDy). We investigated the time during the follicular phase when these cells become activated (i.e., co-localise c-Fos) relative to the timing of the LH surge onset and may therefore be involved in the surge generating mechanism. Furthermore, we aimed to elucidate whether these activation patterns are altered after lipopolysaccharide (LPS) administration, which is known to inhibit the LH surge. Follicular phases of ewes were synchronised by progesterone withdrawal and blood samples were collected every 2 h. Hypothalamic tissue was retrieved at various times during the follicular phase with or without the administration of LPS (100 ng/kg). The percentage of activated dopamine cells decreased before the onset of sexual behaviour, whereas activation of β-endorphin decreased and NPY activation tended to increase during the LH surge. These patterns were not disturbed by LPS administration. Maximal co-expression of c-Fos in dynorphin immunoreactive neurons was observed earlier during the follicular phase, compared to kisspeptin and NKB, which were maximally activated during the surge. This indicates a distinct role for ARC dynorphin in the LH surge generation mechanism. Acute LPS decreased the percentage of activated dynorphin and kisspeptin immunoreactive cells. Thus, in the ovary-intact ewe, KNDy neurones are activated prior to the LH surge onset and this pattern is inhibited by the administration of LPS.

Activation of cells containing estrogen receptor alpha or somatostatin in the medial preoptic area, arcuate nucleus, and ventromedial nucleus of intact ewes during the follicular phase, and alteration after lipopolysaccharide

Cells in the medial preoptic area (mPOA), arcuate nucleus (ARC), and ventromedial nucleus (VMN) that possess estrogen receptor alpha (ER alpha) mediate estradiol feedback to regulate endocrine and behavioral events during the estrous cycle. A percentage of ER alpha cells located in the ARC and VMN express somatostatin (SST) and are activated in response to estradiol. The aims of the present study were to investigate the location of c-Fos, a marker for activation, in cells containing ER alpha or SST at various times during the follicular phase and to determine whether lipopolysaccharide (LPS) administration, which leads to disruption of the luteinizing hormone (LH) surge, is accompanied by altered ER alpha and/or SST activation patterns. Follicular phases were synchronized with progesterone vaginal pessaries, and control animals were killed at 0, 16, 31, and 40 h (n = 4-6/group) after progesterone withdrawal (PW [time 0]). At 28 h, other animals received LPS (100 ng/kg) and were subsequently killed at 31 h or 40 h (n = 5/group). Hypothalamic sections were immunostained for c-Fos and ER alpha or SST. LH surges occurred only in control ewes with onset at 36.7 ± 1.3 h after PW; these animals had a marked increase in the percentage of ER alpha cells that colocalized c-Fos (%ER alpha/c-Fos) in the ARC and mPOA from 31 h after PW and throughout the LH surge. In the VMN, %ER alpha/c-Fos was higher in animals that expressed sexual behavior than in those that did not. SST cell activation in the ARC and VMN was greater during the LH surge than in other stages in the follicular phase. At 31 or 40 h after PW (i.e., 3 or 12 h after treatment, respectively), LPS decreased %ER alpha/c-Fos in the ARC and the mPOA, but there was no change in the VMN compared to that in controls. The %SST/c-Fos increased in the VMN at 31 h after PW (i.e., 3 h after LPS) with no change in the ARC compared to controls. These results indicate that there is a distinct temporal pattern of ER alpha cell activation in the hypothalamus during the follicular phase, which begins in the ARC and mPOA at least 6-7 h before the LH surge onset and extends to the VMN after the onset of sexual behavior and LH surge. Furthermore, during the surge, some of these ER alpha-activated cells may be SST-secreting cells. This pattern is markedly altered by LPS administered during the late follicular phase, indicating that the disruptive effects of this stressor are mediated by suppressing ER alpha cell activation at the level of the mPOA and ARC and enhancing SST cell activation in the VMN, leading to the attenuation of the LH surge.

Effects of estrogen on growth hormone pulsatility in peripheral blood and neuropeptide profiles in the cerebrospinal fluid of goats

We previously reported that growth hormone (GH) pulses were negatively associated with neuropeptide Y (NPY) profiles in cerebrospinal fluid (CSF) of the third ventricle of Shiba goats. In addition, while most GH pulses were coincident with GH-releasing hormone (GHRH) pulses, there was no correlation between GH and somatostatin (SRIF) levels. The present study was performed to elucidate the relationship between GH pulses and these neuropeptide levels in CSF when estradiol (1.0 mg/head) was subcutaneously administered to ovariectomized goats. CSF and plasma samples were collected every 15 min for 18 h (from 6 h before to 12 h after injection). GH levels in peripheral blood and GHRH, SRIF and NPY levels in CSF were measured by radioimmunoassay. Pulse/trough characteristics and correlations were assessed by the ULTRA algorithm and cross-correlation analysis. Before estradiol was injected, significant coincidence was found between GHRH pulses and GH pulses, and negative coincidence was found between NPY troughs and GH pulses. Six to 12 h after estradiol injection, the amplitude and area under the curve (AUC) of the GH pulses were markedly increased. The duration and AUC of the GHRH pulses in the CSF were also increased, and stronger synchrony of GHRH with GH was observed. In contrast, the baseline of NPY was significantly decreased, and the negative correlation between the GH pulses and NPY troughs disappeared. The parameters of SRIF troughs were not clearly changed. These observations suggest that estrogen enhances the pattern of secretion of GH in the goat via enhancement of GHRH pulses and decrease of NPY levels.

Reproduction phase-related expression of beta-endorphin-like immunoreactivity in the nucleus lateralis tuberis of the female Indian major carp Cirrhinus mrigala: correlation with the luteinising hormone cells-ovary axis

The present study aimed to determine whether beta-endorphin immunoreactivity (bEP-ir) in the neurones of the nucleus lateralis tuberis (NLT) is linked to the seasonal cycle and shows correlation with the number of luteinising hormone (LH) cells in the pituitary gland and ovaries in the teleost, Cirrhinus mrigala. Although LH cells were moderately immunostained during the resting phase (December to January), the morphological profile suggested increased synthetic and secretory activity during the preparatory (February to April) and prespawning (May to June) phases. However, LH immunoreactivity was greatly reduced (P < 0.001) in the spawning (July to August) phase, suggesting massive discharge of the hormone; this pool was partly replenished in the postspawning (September to November) phase. The ovaries grew rapidly in the preparatory and prespawning phases; maximal size was attained during spawning, when ovulation occurred. Thereafter, the ovaries regressed. The NLT of C. mrigala is divisible into the pars lateralis (NLTl) and medialis (NLTm). During the postspawning and resting phases, bEP-ir was readily detectable in the NLTm as well as NLTl neurones. However, a steady reduction in the immunoreactivity was observed in the NLTm neurones during the preparatory through spawning phases (P < 0.001), suggesting a negative correlation with the LH cells-ovary axis. Thus, the inhibitory influence of beta-endorphin on the gonadotrophin-releasing hormone (GnRH)-LH axis appears to be attenuated during the preparatory through spawning phases. This may be necessary for the rapid stimulation of the axis culminating in spawning. Neurones of the NLTl also showed a gradual reduction in bEP-ir during the preparatory and prespawning phases (P < 0.01) and may therefore play a similar role. However, significant augmentation of the immunoreactivity was noticed in these neurones during the spawning phase (P < 0.001), the physiological significance of which is unknown. Although the present study demonstrated a temporal correlation between the beta-endorphin in the NLT, LH cells and the ovary, we suggest that the peptide in the NLTl and NLTm may show functional duality during the spawning phase.

Neuropeptide Y (NPY) delays the oestrogen-induced luteinizing hormone (LH) surge in the ovariectomized ewe: further evidence that NPY has a predominant negative effect on LH secretion in the ewe

Studies in rats suggest that neuropeptide Y (NPY) plays a stimulatory role in the generation of the preovulatory luteinizing hormone (LH) surge, via the Y1 receptor. We have investigated this issue using the oestradiol benzoate (EB)-treated ovariectomized (OVX) ewe which is a model for the preovulatory LH surge. A Y1 receptor antagonist (BIBO3304) was infused (25 microg/h) into the third cerebral ventricle (III-V) from 2 h before EB injection for 24 h, and had no effect on the ensuing LH surge. Using in situ hybridization, we then examined expression of NPY mRNA in the arcuate nucleus during the luteal, follicular and oestrous phases of the oestrous cycle, and found that levels were greatest during the luteal phase. Thus, reduced NPY synthesis might be an integral factor in the events leading to the cyclic preovulatory LH surge. This was tested by infusion of NPY (25 microg/h) into the III-V (as above). The NPY infusion delayed the LH surge until the infusion was ceased. High levels of NPY expression during the luteal phase of the oestrous cycle may be caused by progesterone. Thus, we determined whether NPY cells possess progesterone receptors (PR) and whether progesterone treatment up-regulates NPY mRNA expression in the arcuate nucleus. Immunohistochemistry for NPY and PR was performed in OVX, oestrogen-treated ewes, but no NPY cells of the arcuate nucleus were seen to colocalize PR. In situ hybridization for NPY was performed in OVX and OVX ewes treated with progesterone. There was no significant effect of progesterone treatment on NPY mRNA expression in the arcuate nucleus. We conclude that chronically elevated levels of NPY block the preovulatory surge of gonadotropin-releasing hormone/LH secretion in sheep, but high levels of NPY mRNA expression in the luteal phase of the oestrous cycle cannot be explained by an action of progesterone.

Seasonal changes in the inputs to gonadotropin-releasing hormone neurones in the ewe brain: an assessment by conventional fluorescence and confocal microscopy

The seasonal pattern of breeding in sheep offers an opportunity to examine plasticity of neuronal inputs to gonadotropin-releasing hormone (GnRH) neurones. We used conventional fluorescence microscopy and confocal microscopy to compare the extent of input to GnRH neurones from various neuropeptide/neurotransmitter systems in ewes during the breeding and anestrous seasons. Using double-labelling immunohistochemistry, we counted close appositions between GnRH cells and varicosities that were immunoreactive for either glutamic acid decarboxylase (GAD; for gamma-amino butyric acid-GABA-neurones), dopamine beta hydroxylase (DBH; for noradrenergic neurones), vesicular glutamate transporter-1 (VGluT-1, for glutamatergic neurones), neuropeptide Y (NPY) and tyrosine hydroxylase (TH; for dopaminergic/noradrenergic neurones). The percentage of GnRH cells displaying close appositions to GABA-ergic varicosities was higher (P < 0.02) in anestrus than in the breeding season. The percentage of GnRH cells receiving input from varicosities that were positive for TH, DBH and VGluT-1 was similar in both seasons. Approximately 26-49% of GnRH neurones were seen to receive inputs from NPY, TH, GABAergic or noradrenergic neurones, while a larger number of GnRH cells (72-75%) received input from glutamatergic neurones. Conventional microscopy consistently overestimated the number of close contacts on GnRH neurones compared to confocal microscopy. For TH-immunoreactive varicosities in the preoptic area, only 16-35% were also immunoreactive for DBH, suggesting that the remainder are dopaminergic. Approximately half of the noradrenergic inputs in the preoptic area were also immunoreactive for NPY. In conclusion, we present numerical data on the consensus between light and confocal microscopy and the level of input of various neuronal systems to GnRH cells; the data indicate a seasonal change in the GABAergic input to GnRH neurones.

Effect of food manipulation on the neuropeptide Y neuronal system in the diencephalon of ewes

The expression of neuropeptide Y (NPY), which is involved in the neuromodulatory function associated with animal nutrition, growth and reproduction, depends on the nutritional status. However, the roles of individual components of food are not fully recognised. In this study, we evaluated the effects of dietary protein levels on the diencephalic population of NPY neurones. Female lambs were fed two diets equilibrated energetically but containing 8% protein in restricted diet or 18% protein in elevated diet, for 15 weeks starting with 6 months of age, during the first breeding period. Then, brain tissues were collected, fixed and used for the immunohistochemical localisation of NPY. Detection of NPY in diencephalon sections was followed by the image analysis and expressed as the percent area stained and optical density of immunostaining. Two distinct populations of the immunoreactive NPY perikarya were found, one in the infundibular nucleus; and the other in areas of diencephalon adjacent to the rostral hypothalamus. Long-term feeding the protein restricted diet caused a prominent expression of the immunoreactive material solely in the hypothalamic NPY neurones, particularly in those located in the entire periventricular area and in the infundibular nucleus. Both, percent area exhibiting positive staining and the density of immunoreactive NPY measured in the periventricular subdivision of the paraventricular nucleus were significantly higher (P<0.05) in sheep fed restricted diet, than in sheep fed on elevated diet. This study describes the distribution of NPY neurones in the sheep diencephalon, and shows the relationships between the expression of hypothalamic population of NPY neurones and the level of protein feeding.

Effect of season on neuropeptide Y and galanin within the hypothalamus of the ewe in relation to plasma luteinizing hormone concentrations and the breeding season: an immunohistochemical analysis

Within the hypothalamus, neurones that express neuropeptide Y (NPY) and galanin have been implicated in the regulation of gonadotropin-releasing hormone (GnRH) and gonadotropin secretion. We aimed to determine the extent to which the expression of these two neuronal systems is linked to the seasonal reproductive cycle, and the effect of chronic oestrogen treatment. Immunohistochemical analysis was used to examine changes between the breeding season and anestrus in ovariectomized (OVX) ewes with or without oestrogen treatment (s.c. implants for 2 weeks). Serial blood sampling established plasma luteinizing hormone (LH) profiles, and the ewes were subsequently killed and the brains perfused for immunohistochemistry. In OVX ewes, the amplitude of LH pulses was greater in the nonbreeding season than in the breeding season. Oestrogen treatment caused a marked reduction in plasma LH concentrations during anestrus, but not in the breeding season. The number of cells in the arcuate nucleus/median eminence region (ARC-ME) that stained for NPY was lower in ewes killed in anestrus (September) than in ewes killed in the breeding season (May), but there was no seasonal change in the number of galanin-stained cells. Within season, oestrogen treatment did not affect NPY- or galanin-cell number. There was no effect of season or oestrogen on the area of varicose fibres/terminals for either peptide in the ARC-ME, but galanin immunostaining was more intense during the breeding season. We conclude that the amount of NPY in cell bodies of the ARC-ME is lower in ewes in the nonbreeding season; this could reflect a steroid-independent effect of photoperiod. We also conclude that the long-term negative-feedback effect of oestrogen on GnRH/LH secretion does not appear to be mediated by NPY- or galanin-containing neurones in the ewe.

Distribution of galanin immunoreactivity in the sheep diencephalon

Although the physiological role of galanin has been demonstrated in several endocrine regulations in sheep, the anatomical characteristics of this neuronal system has never been studied. The distribution of galanin-containing neurones was described by immunohistochemistry using galanin antiserum in the diencephalon of adult ewes, both ovariectomized or treated with colchicine. Galanin-immunoreactivity was found throughout the diencephalon. In the ovariectomized ewes, galanin-immunoreactive neurones were mainly observed in the medial preoptic area and the infundibular nucleus. The highest density of immunoreactive fibres was found in the external layer of the median eminence. Numerous galanin-immunoreactive fibres were also observed in the preoptic area, the mediobasal hypothalamus, the periphery of the supraoptic and the paraventricular nuclei. With colchicine treatment, the number of labelled neurones increased, and additional galanin-immunoreactive perikarya were observed in the bed nucleus of the stria terminalis, the lateral septum, the supraoptic, the paraventricular and the periventricular nuclei and the paraventricular nucleus of the thalamus. In the caudal part of the diencephalon, the density of labelled neurones was lower in both groups of animals than in other species studied. Regardless of treatment, labelling was not seen in the suprachiasmatic nucleus and only rarely in the ventromedial nucleus. These results describe, for the first time, the distribution of galanin-immunoreactive neurones in the sheep diencephalon. Compared to other species studied, distribution in the sheep diencephalon has several distinct differences. In ovariectomized animals, the medial preoptic area presents more labelled neurones in sheep than in monkeys, whereas in the supraoptic nucleus the density of labelled neurones is lower in sheep than in humans or opossums. After colchicine treatment only very few differences were observed between sheep and rats, but in contrast to other species, the suprachiasmatic nucleus of the sheep does not contain labelled neurones.

Presence of galanin in dopaminergic neurons of the sheep infundibular nucleus: a double staining immunohistochemical study

The distribution of tyrosine hydroxylase (TH) and of galanin immunoreactive (IR) neurons were examined in the sheep infundibular nucleus. Antisera raised against TH and galanin were used on adjacent sections and for double immunohistochemical staining of the same sections. There was considerable overlap in the distribution of TH and galanin-IR neurons in the medial part of the nucleus. Most of the galanin-IR neurons were also TH-IR, but less than 50% of the TH-IR neurons also expressed galanin immunoreactivity. Neurons immunoreactive to TH alone were observed close to the third ventricle and in the rostral part of the infundibular nucleus. In the median eminence, TH and galanin-IR fibres overlapped mainly in the lateral and dorsal parts of the external layer, but the colocalisation of both antigens could not be assessed on the available material. Thus, in sheep, the population of catecholaminergic neurons of the infundibular nucleus may be subdivided into different subpopulations according to their peptide content, but does not appear segregated as in rat and human.

Effects of increasing gestation, cortisol and maternal undernutrition on hypothalamic neuropeptide Y expression in the sheep fetus

We have characterized the localization and the ontogenetic changes in Neuropeptide tyrosine (NPY) before birth and investigated the regulation of NPY expression by cortisol and undernutrition in the fetal sheep hypothalamus during late gestation. Using immunohistochemistry, we have identified NPY-containing neurons in the infundibular nucleus and the internal layer of the median eminence in fetal hypothalami collected between 110 and 147 days gestation. NPY projections were also present in the paraventricular nucleus (PVN) of fetal hypothalami at all ages between 110 days gestation and term. There was a significant increase in the amount of immunoreactive NPY/g hypothalamus between 87 and 113 days and 131-140 days gestation and a further significant increase after 141 days gestation. The total hypothalamic content of immunoreactive NPY increased significantly between 87 and 113 days and 141-145 days gestation. The levels of NPY mRNA: 18S rRNA in the mediobasal region of the fetal hypothalamus were significantly higher at 145-146 days gestation than at any earlier gestational age between 116 and 141 days gestation. Cortisol (2.5-3.0 mg/24 h) was infused intrafetally between 109 and 116 days gestation. The ratio of NPY mRNA: 18s rRNA in the mediobasal region of the fetal hypothalamus was significantly higher in the cortisol-infused group when compared with the saline-infused control group at 116 days gestation. Maternal, and hence fetal undernutrition, was induced between 110 and 146 days gestation. At 145-146 days gestation the ratio of NPY mRNA: 18S rRNA in the mediobasal region of the fetal hypothalamus was significantly higher in the undernutrition group when compared with control animals. We have therefore demonstrated that NPY is present in the hypothalamus of the sheep fetus before birth and that hypothalamic NPY content and NPY mRNA increase before delivery. We have also found that glucocorticoids and undernutrition stimulate increases in NPY mRNA levels in the hypothalamus before birth.

Localization of the preprosomatostatin-mRNA by in situ hybridization in the ewe hypothalamus

The peptidergic neurohormone somatostatin (SRIF) derives from a precursor called preprosomatostatin (PPS) by proteolysis. We have isolated by RT-PCR and sequenced a partial cDNA coding for the ovine PPS. It contains a 348 base pairs coding sequence that shares strong similarities with previously cloned mammalian cDNAs. The ovine cDNA was used to synthesize radiolabeled cRNA to probe the PPS mRNA in the ewe hypothalamus by in situ hybridization. The PPS mRNA-containing cells are widely distributed in the hypothalamus. According to the number of silver grains over a cell, they show various staining intensities. The distribution of the PPS mRNA is in good agreement with that of the peptide previously determined using immunohistochemistry. The strongest labeled areas include the periventricular region of the paraventricular nucleus and the lateral division of the ventromedial nucleus. The difference in labeling intensity observed in the diverse populations of labeled neurons could reflect various levels of neuronal activity.

A subset of estrogen receptor-containing neurons project to the median eminence in the ewe

The neural pathways responsible for conveying the steroid feedback signals that ultimately affect reproductive neuroendocrine function remain largely undefined. One possibility involves a direct projection from estrogen receptor (ER)-containing neurons to the median eminence (ME), a site of neuroendocrine peptide release. To examine this possibility, 8 ewes received stereotaxic injections of the retrograde neuronal tract-tracing compound cholera toxin-beta subunit (CT beta) into the ME. Neurons sending projections to the ME and containing ER were identified using a dual-label immunoperoxidase method. Double-labeled cells were found in distinct regions: (1) the ER-rich arcuate nucleus (ARC) that contained the greatest number of double-labeled cells, and (2) the organum vasculosum of the lamina terminalis (OVLT) which contained a very consistent, but low, number of double-labeled cells. While a fairly large number of retrogradely-labeled ARC neurons containing ER were identified, the majority of ER-containing ARC neurons were unlabeled and thus send projections elsewhere. Other regions containing high concentrations of ER-positive cells such as the medial preoptic area (MPOA), anterior hypothalamic area, and ventrolateral portion of the ventromedial hypothalamic nucleus, were devoid of double-labeled cells. Similarly, regions rich in neuroendocrine neurons such as the periventricular hypothalamus and paraventricular and supraoptic hypothalamic nuclei contained no double-labeled cells. These results suggest that modulation of neuroendocrine secretory activity may occur directly at the level of the ME by ER-containing neurons located within restricted regions of the hypothalamus and forebrain. However, the relatively low proportion of ER-containing neurons projecting to the ME suggests that the influence of estradiol upon neuroendocrine function also may include target sites other than the ME.

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

Anterograde tracers, viz. Phaseolus vulgaris leucoagglutinin and fluorescein dextran, were used in conjunction with tyrosine hydroxylase immunohistochemistry to study the projections of the A15 dopaminergic cell group towards the median eminence and pituitary in sheep. After injection of the tracers in the retrochiasmatic area, which contains the cell group A15, fibres containing anterograde tracer were observed in the internal zone of the median eminence and in the pars nervosa of the pituitary. Numerous tyrosine hydroxylase immunoreactive fibres were present in the external zone of the median eminence and in the pars intermedia and the pars nervosa of the pituitary, with characteristic patterns of organisation in each area. Most tyrosine hydroxylase-immunoreactive fibres containing fluorescein dextran were located in the pars nervosa, whereas only a few were observed in the internal zone of the median eminence. It was concluded that at least part of the dopaminergic innervation of the pars nervosa originated from the A15 group. These results provide morphological evidence for (1) the role of dopaminergic neurons of the A15 cell group in the seasonal control of prolactin secretion via the release of dopamine in the pars nervosa, and (2) putative physiological interactions between dopamine and the secretion of neurohypophysial hormones in sheep.

Neuronal projections to the medial preoptic area of the sheep, with special reference to monoaminergic afferents: immunohistochemical and retrograde tract tracing studies

The preoptic area contains most of the luteinizing hormone releasing hormone immunoreactive neurons and numerous monoaminergic afferents whose 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 medial preoptic area in sheep. Among the retrogradely labeled neurons, immunohistochemistry for tyrosine hydroxylase, dopamine-beta-hydroxylase, phenylethanolamine N-methyltransferase, and serotonin was used to characterize catecholamine and serotonin fluorogold labeled neurons. Most of the afferents came from the ipsilateral side to the injection site. It was observed that the medial preoptic area received major inputs from the diagonal band of Broca, the lateral septum, the thalamic paraventricular nucleus, the lateral hypothalamus, the area dorsolateral to the third ventricle, the perimamillary area, the amygdala, and the ventral part of the hippocampus. Other numerous, scattered, retrogradely labeled neurons were observed in the ventral part of the preoptic area, the vascular organ of the lamina terminalis, the ventromedial part of the hypothalamus, the periventricular area, the area lateral to the interpeduncular nucleus, and the dorsal vagal complex. Noradrenergic afferents came from the complex of the locus coeruleus (A6/A7 groups) and from the ventro-lateral medulla (group A1). However, dopaminergic and adrenergic neuronal groups retrogradely labeled with fluorogold were not observed. Serotoninergic fluorogold labeled neurons belonged to the medial raphe nucleus (B8, B5) and to the serotoninergic group situated lateral to the interpeduncular nucleus (S4). In the light of these anatomical data we hypothesize that these afferents have a role in the regulation of several functions of the preoptic area, particularly those related to reproduction. Accordingly these afferents could be involved in the control of luteinizing hormone releasing hormone (LHRH) pulsatility or of preovulatory LHRH surge.

Immunohistochemical colocalization of tyrosine hydroxylase and estradiol receptors in the sheep arcuate nucleus

In sheep, the arcuate nucleus contains numerous tyrosine hydroxylase (TH) and estradiol receptor (rE2) immunoreactive (IR) perikarya and it has been shown previously in this species that catecholaminergic neurons can mediate the gonadal steroid action on the reproductive function. In the present study, double immunohistochemical labelling with antibodies against TH and rE2 have been used to demonstrate the presence of rE2 in TH-IR neurons in the arcuate nucleus where the distribution of TH-IR and rE2-IR neurons overlap each other. Only less than 10% of all the rE2-IR perikarya presented TH immunoreactivity. It was therefore hypothesized that either such a low number of double labelled neurons can support the effects of estradiol in this area or that the effect of this steroid was indirect. In the latter case it might be first mediated by beta-endorphin neurons which have been previously described in this nucleus.

Acute changes in growth hormone-releasing hormone secretion after injection of BIM 23014, a long acting somatostatin analog, in rams

BIM 23014 is a somatostatin analog displaying an increased biological half life due to resistance to enzymatic degradation. This peptide inhibits GH release directly at the level of pituitary somatotrophs. In addition, an action of BIM 23014 at the level of the hypothalamus is possible since somatostatinergic fibers and receptors have been identified on GH-RH neurons. To evaluate the effect of BIM 23014 on GH-RH secretion, hypophysial portal blood (HPB) was continuously collected in conscious sheep. Twelve rams (40-45 kg, 9-month-old) with chronically implanted perihypophysial cannulae were i.v. injected with BIM 23014 (1 mg) or saline. HPB and jugular blood were collected for 3-5 hours before and after the injection for the determinations of GH-RH and GH concentrations respectively. The acute injection of BIM 23014 induced a rapid decrease of plasma GH within the first two hours. Simultaneously, GH-RH in HPB decreased significantly. After reaching a nadir, GH concentrations increased to values greater than baseline. A similar rebound in GH-RH levels in HPB was also observed. These data indicate that BIM 23014 acts at the level of GH-RH hypothalamic neurons, in addition to its well-know effect on the pituitary gland.