Pharmacological characterisation of oxytocin binding sites in the ovine pineal gland.

Both oxytocin (OT) and [Arg8]vasopressin (AVP) are found within the ovine pineal gland and may function to modulate melatonin secretion. However, the receptors which mediate the actions of these peptides have yet to be characterised. Preliminary studies of ovine pineal microsomal cell membranes showed binding of [3H]OT (79+/-9 fmol/mg) 10 times greater than binding of [3H]AVP (8+/-3 fmol/mg). Saturation studies using either [3H]OT or the selective OT receptor ligand [125I]d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin (OTA) revealed high affinity, single site kinetics (Kd = 1.72+/-0.32 nM; Bmax = 68+/-18 fmol/mg). Binding of [3H]AVP was more effectively displaced by OT than AVP, suggesting that binding may be due to cross-reaction with the OT binding site. Displacement of [3H]OT using a range of selective agonists and antagonist analogues revealed pharmacological characteristics similar to [3H]OT binding sites in the ovine and rat uterus. These data show that the ovine pineal expresses a high density of OT binding sites which may participate in the regulation of melatonin secretion.

Single dose pharmacokinetics of medetomidine in sheep.

The pharmacokinetics of medetomidine hydrochloride (Domitor) administered at a single dose of 15 micrograms/kg i.v. in sheep are described. Plasma medetomidine concentrations were determined using a sensitive radioreceptor assay technique, capable of also measuring metabolites which would bind to alpha 2 adrenergic receptors. Medetomidine was rapidly distributed, with a half-life of distribution of 4.65 +/- 0.65 min. The apparent volume of distribution was 2.69 +/- 0.62 L/kg, while elimination half-life was 37.85 +/- 2.84 min. Total body clearance varied between 16.29 and 151.81 mL/min.kg. Pharmacological effects of medetomidine paralleled its plasma concentration.

Radioreceptor assay for determination of xylazine and medetomidine in sheep plasma.

A radioreceptor assay technique is described for the measurement of xylazine and medetomidine in sheep plasma. The assay was based on the displacement of tritiated clonidine from a2-adrenoceptors in a rat brain homogenate by xylazine or medetomidine extracted from plasma. Plasma samples from sheep which had been given xylazine and medetomidine were treated with alumina to remove endogenous catecholamines which would otherwise have bound to alpha 2-adrenoceptors and interfered with the assay. The drugs were then extracted using chloroform, reconstituted in buffer and used to displace [3H]clonidine. The concentration of alpha 2-agonist was calculated by reference to standard curves. The method had a detection limit of 2.5 ng/mL for xylazine and 0.24 ng/mL for medetomidine. The assay could also be used to detect metabolites capable of binding to alpha 2-receptors.