A solid-phase radioimmunoassay for ovine luteinizing hormone (LH) has been developed, utilizing antibody-coated polystyrene tubes for incubation of the assay and counting of the bound tracer. Tubes were coated with equine antiserum to bovine LH (Snook), and purified ovine LH (Papkoff) was used for iodination with 126 I and assay standards. The procedure is simple and rapid, being completely performed in the assay tubes and giving results after a period of 48 hr. Basal levels of plasma LH were 2.9 ±0.9 ng/ml in the cycling ewe, 1.2 ±0.9 ng/ml in the ram, 12.6 ±5.5 ng/ml in the oophorectomized ewe, and 1.2 ±0.9 ng/ml in the pregnant ewe. These values obtained from jugular venous plasma are 25% higher than those measured in peripheral venous plasma. The sheep does not appear to produce a placental gonadotrophin comparable to human chorionic gonadotrophin in regard to immunological crossreaction with pituitary LH. Plasma levels of LH rose sharply 4-16 hr after the onset of estrus to levels of 80-200 ng/ml, over a total period of only 10 hr. Frequent blood sampling is necessary to delineate the estrous LH release in the sheep, and daily estimations carry a greater than 50% chance of completely missing the LH peak. Administration of estradiol-17/3 to anestrous sheep by intramuscular injection and intravenous infusion was regularly followed by a typical estrous peak of LH secretion. The latent period of the estrogen stimulus was approximately 9 hr, and the dose required to produce an ovulatory LH peak was 6-10 ng, an amount similar to that secreted by the ovary at estrus. It is likely that estrogen secretion by the ovary is a major factor in stimulating the LH release accompanying estrus in the sheep (Endocrinology 85: 133, 1969) T HE STUDY of pituitary gonadotrophin secretion in the sheep was previously dependent upon indirect observations and the use of bioassay techniques. The timing of luteinizing hormone (LH) release has been examined by estimation of pituitary LH content (1) and by measurement of LH in blood obtained from the cavernous sinus (2). These studies have relied upon ovarian ascorbic acid depletion assays (3, 4) for quantitation of LH, a procedure which is adequate for the measurement of pituitary LH content but not generally regarded as satisfactory for the assay of blood LH levels. A decrease in pituitary LH content may indicate release of LH into the circulation, though this is uncertain unless the rate of hormone synthesis is also known, while cavernous sinus

1 Neurotransmitters released from nerve endings are inactivated by re-uptake into the presynaptic nerve terminals and possibly into neighbouring glial cells. While analysing the functional properties of a,-adrenoceptors in the hypothalamus, we observed a high-affinity uptake process for noradrenaline in postsynaptic peptidergic neurones.2 In primary hypothalamic cell cultures and in a hypothalamic neuronal cell line, [3H]-prazosin bound with high affinity and was displaced by unlabelled prazosin in concentrations of 10-10 to 10-7M. However, at concentrations of unlabelled prazosin above 10-7 M, there was a paradoxical increase in 9 The measured uptake of (-)-noradrenaline in the cell line was considerably increased by blockade of catechol-O-methyl-transferase and monoamine oxidase, suggesting that (-)-noradrenaline is metabolized to lipophilic products that escape across the plasma membrane. 10 Studies in rats, in which the noradrenaline isomer 6-hydroxydopamine was used, suggested that the postsynaptic uptake process is operative in hypothalamic CRH and vasopressin neurones in vivo.11 The Km for (-)-noradrenaline was within the range for the high affinity uptake, process in noradrenergic neurones. Uptake takes place in concentrations at which noradrenaline activates oxtadrenoceptors. Removal of noradrenaline from the vicinity of the receptors may prevent desensitization, thus maintaining the responsiveness of postsynaptic neurones to the actions of the neurotransmitter.

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