Previous studies have shown that there is a population of noradrenergic cells in the caudal A1 field of the brainstem of the ewe that contain oestrogen receptors and project to the preoptic area, where gonadotrophin releasing hormone (GnRH) neurones are located. There are some discrepancies in the literature regarding the extent of this projection and the location of the cells in the A1 region. The issue has been a focus of attention because the positive feedback response to oestrogen that causes the ovulatory GnRH/luteinizing hormone surge may originate from this brainstem region. The aim of the present study was to determine the extent of the projections to the preoptic area and to determine whether the caudal A1 cells are activated by oestrogen. Eleven ovariectomized ewes received an injection of the retrograde tracer FluoroGold into the preoptic hypothalamus and four of these also received an i.m. injection of oestrogen 2 h before tissue collection. A further three sheep received i.m. oil injections to act as controls for those receiving oestrogen. Dopamine-beta-hydroxylase (DBH)-positive, retrogradely labelled cells were found within the A1 field in sheep that received preoptic FluoroGold injections. Cells in the vicinity of the A2 and A6 fields, that were retrogradely labelled with FluoroGold, were not DBH-positive. Thus, cells in the A1 field provide a direct noradrenergic projection to the preoptic area and may be involved in the control of the secretion of GnRH in this species. Cells that project to the preoptic hypothalamus from more rostrally located areas of the brainstem are not noradrenergic. In the animals that received oestrogen, double-labelling immunohistochemistry was performed throughout the A1 field for FluoroGold, DBH and Fos. DBH cells of the A1 field expressed Fos only in the oestrogen-treated animals and not in the oil-treated animals. There was a decline in the number of DBH cells that were retrogradely labelled from the caudal region of A1 towards obex. There was a similar gradient in the number of cells that were double-labelled for Fos and FluoroGold. We conclude that there is a population of noradrenergic cells in the caudal A1 field that project to the preoptic area; this is a larger group of cells than previously reported. Oestrogen elicits an acute Fos response in these cells, which may be involved in the time-delayed positive feedback response on GnRH cells. The caudal-to-rostral gradient in the labelling with FluoroGold and Fos in DBH-positive cells is similar to that seen previously for oestrogen receptor in DBH-positive cells in the A1 field.

Oestrogen produces a positive feedback effect on the secretion of gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH) when implanted into the ventromedial/arcuate nucleus of the ovariectomized (OVX) ewe. This has led to the belief that it is in this area of the hypothalamus that oestrogen causes the preovulatory surge in GnRH/LH. To date, however, the cell types that are integral to this response have not been identified. The present study aimed to examine cellular responsiveness to oestrogen in this region of the brain using Fos immunohistochemistry and further aimed to determine the cell type that shows an acute response to oestrogen. OVX ewes (n = 4-6 per group) were given i.m. injections of oestradiol benzoate or oil (vehicle) and were killed 1-6 h later. Brains were perfused for immunohistochemistry. The number of cells in the arcuate nucleus which were immunopositive for Fos was greater (two- to fourfold) in the oestradiol benzoate-treated OVX ewes (n = 5) 1 h after injection. The number of Fos-positive cells in the ventromedial hypothalamic nucleus was 10-fold greater in the oestradiol benzoate-treated ewes 1 h after injection. Because there were high levels of Fos-immunoreactive cells in oil-treated ewes, we repeated the experiment with i.v. injection of 50 microg oestrogen or vehicle (n = 5). With this latter procedure, we found that oestrogen injection caused a significant increase in the number of Fos immunoreactive cells in the arcuate nucleus within 1 h, but there was no response in the ventromedial hypothalamus. To further characterize the types of cells that might respond to oestrogen, we double-labelled cells for Fos and either adrenocorticotropin hormone, neuropeptide Y or tyrosine hydroxylase (a marker for dopaminergic cells). These cell types could account for less than 30% of the total number of cells that were Fos-positive and oestrogen treatment did not cause an increase in the Fos labelling of any of these types of cell. These data show that oestrogen activates cells of the arcuate/ventromedial hypothalamus within 1 h of injection and that this response could relate to the feedback effects of this gonadal hormone. The majority of cells that produce Fos following oestrogen injection are of unknown phenotype. The data further suggest that induction of cells of the ventromedial hypothalamic nucleus require more prolonged oestrogen stimulus than cells of the arcuate nucleus.

ABSTRACI'. Our aim was to determine the effects of oligohydramnios during the last third of ovinc gestation on respiratory function in lambs during their first postnatal month. To induce oligohydramnios, amniotic and allantoic fluids were drained from pregnant ewes, starting at 109.0 +. 2.3 d of pregnancy (term = 148 d). In 10 lambs born at term, respiratory function was studied four times at rveeklg intervals; a group of nine lambs from normal pregnancies served a s controls. Over the 4-wk study period, treated lambs had significantly higher breathing rates and snlallcr tidal volumcs than controls, although the differences diminished with age. hlinutc ventilation and O2 consumption were the same in each group, and when related to body weight, both declined with age. Treated lambs were normoscmic but were hypercapnic compared with controls for up to 4 wk. Functional residual capacity, measured by helium dilution, was the same in each group and increased with age. Static compliance of the rcspiratory system was lower in treated lambs up to 4 wk; lung compliances were the same in each group, but chest wall compliance was lower in treated lambs than in controls for 4 w k. Postmortem measurements, at 27-28 d, of pulmonary dry weights, DNA contents, and protein contents suggest tliat the lungs of treated lambs may have bcen mildly hypoplastic. \\'e conclude tliat oligohydramnios causes a decreased chest wall compliance, which lcads to rapid, shallo\v breathing and a mild hypercapnia lasting for at least 4 postnatal wk.

We studied the effects of postnatal age on arousal and cardiorespiratory responses to airflow obstruction in sleeping lambs: we also determined the influence of sleep states and repeated airflow obstruction. Sixteen lambs were chronically prepared for monitoring sleep states, arterial O2 saturation (SaO2), heart rate (HR), and intrapleural pressure (Pp1) and were studied from 2-29 days after birth. Obstruction of respiratory airflow by facemask occlusion led to arterial desaturation, augmentation of respiratory efforts, bradycardia, and arousal. Lambs aroused more rapidly and with less desaturation in non-rapid eye movement (NREM) sleep (7 +/- 1 second and 7 +/- 1%, respectively) than in rapid eye movement (REM) sleep (18 +/- 2 seconds and 22 +/- 2%), and cardiac slowing was less in NREM than in REM sleep. In REM sleep only, the arousal latency and desaturation at arousal were affected by postnatal age; arousal responses occurred most rapidly in the youngest (< or = 6 days) and oldest (> or = 13 days) age groups and were delayed at 7-12 days. Repeated episodes of airflow obstruction led to reduced arousability in REM sleep only. We conclude that arousal from REM. but not NREM, sleep in response to the obstruction of respiratory airflow is transiently depressed during early postnatal development and that repeated obstructions and arousals also lead to depressed arousal from REM sleep.

1. Obstruction of the upper airway could be an initiating factor in the Sudden Infant Death Syndrome (SIDS). Responses to upper airway obstruction include augmentation of respiratory efforts, active dilation of the upper airway and electrocortical arousal. Vulnerable individuals may fail to mount these responses effectively. 2. Respiratory and arousal responses to obstruction of the upper airway have been investigated in newborn lambs. In conscious lambs, nasal obstruction causes a profound augmentation of inspiratory efforts, mild asphyxiation and eventual formation of an oral airway. The ability to establish an oral airway involves both chemoreception and mechanoreception and improves with age. 3. In sleeping lambs, obstruction of tidal airflow leads to progressive hypoxaemia, augmentation of inspiratory efforts, bradycardia and arousal. Arousal occurs earlier and with less hypoxaemia and bradycardia in non-REM sleep than in REM sleep. Arousal occurs after inspiratory efforts have increased to the same extent during both sleep states, suggesting that mechanoreception, or a sense of inspiratory effort, is important in initiating arousal. 4. Obstruction of nasal tubes tends to cause arousal from sleep earlier, and with less hypoxaemia and less augmentation of inspiratory effort, than when a more compliant face mask is obstructed. This supports the suggestion that mechanoreception, which may be involved in the perception of inspiratory effort, is a determinant of arousal. 5. With increasing postnatal age, lambs become less arousable in response to airflow obstruction when in REM sleep. This suggests that lambs may become progressively more vulnerable to the effects of airway obstruction during the immediate newborn period.(ABSTRACT TRUNCATED AT 250 WORDS)