Pharmacological characteristics and anatomical distribution of [3H]oxytocin-binding sites in the Wistar rat brain studied by autoradiography

Oxytocinergic innervation of the brain of the garden dormouse (Eliomys quercinus L.)

The oxytocinergic innervation of the brain of the garden dormouse (Eliomys quercinus L.) was studied by means of immunocytochemistry. In contrast to the sparse oxytocin innervation of the rat forebrain, dense fibre networks in various cortical and limbic brain areas were demonstrated in this animal. These include, e.g., the prefrontal cortex, the claustrum, the septum, and the hippocampus. A very dense innervation was also seen in the caudal regions of the garden dormouse brain; these regions are already known to have a relatively dense oxytocin fibre network in the rat. A dense innervation of oxytocin fibres is seen in several brain regions which, in the rat, have oxytocin binding sites but no visible oxytocin innervation. This discrepancy suggests that the differences in the oxytocinergic innervation of these two rodent brains may be due to an oxytocin system in the rat brain that is more difficult to detect immunocytochemically.

Quantitative autoradiographic mapping of neurohypophysial hormone binding sites in the rat forebrain and pituitary gland--II. Comparative study on the Long-Evans and Brattleboro strains

The anatomical distribution and pharmacological characteristics of the different types of neurohypophysial hormone binding sites were compared in the forebrains and pituitary glands of Long-Evans rats and its mutant Brattleboro strain, genetically deficient in vasopressin. Quantitative autoradiography on sections incubated in the presence of 5 nM of either [3H]oxytocin or [3H]vasopressin revealed the presence of the same types of sites in the brains of both strains but noticeable variations in their densities were found in several areas. In the forebrain, oxytocin/vasopressin sites, which bind both peptides with similar high nanomolar affinities, had the same locations and densities in the ventral subiculum, in several nuclei of the amygdala, the bed nucleus of the stria terminalis and the olfactory tubercle. The density of such sites was, in contrast, lower in the ventromedial hypothalamic nucleus of the Brattleboro rat. Selective vasopressin sites which bind [3H]vasopressin with a nanomolar-range affinity and [3H]oxytocin with a much lower affinity showed more variations. They were not found in the Brattleboro rat thalamus but were highly concentrated in several thalamic nuclei in the Long-Evans rat. Conversely, their densities were higher in the dopaminergic A13 cell group of the zona incerta and the suprachiasmatic nucleus of the Brattleboro rat. Their densities were similar in the lateral septal nucleus and in the fundus striati of both strains. In the hypothalamo-neurohypophysial system, [3H]oxytocin and [3H]vasopressin binding occurred in the Long-Evans rat with characteristics different from those found in other brain areas. In the Brattleboro rat, no [3H]vasopressin binding and only low [3H]oxytocin binding, restricted to the magnocellular nuclei, were found.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiographic mapping of neurohypophysial hormone binding sites in the rat forebrain and pituitary gland--I. Characterization of different types of binding sites and their distribution in the Long-Evans strain

Oxytocin and vasopressin binding sites were localized and characterized by quantitative autoradiography on consecutive sections of Long-Evans rat forebrains and pituitary glands, incubated in the presence of 5 nM [3H]oxytocin or 5 nM [3H]vasopressin. In the forebrain, two types of neurohypophysial hormone binding sites were thus defined. (1) Oxytocin/vasopressin sites with similar nanomolar-range affinities for [3H]oxytocin and [3H]vasopressin; both tritiated peptides were displaced from these sites in the presence of 10 microM of either oxytocin or vasopressin. The main areas bearing such sites were the ventral subiculum, several nuclei of the amygdala, the ventromedial hypothalamic nucleus, the bed nucleus of the stria terminalis and the olfactory tubercle. (2) Selective vasopressin sites, binding [3H]vasopressin with nanomolar-range affinity and [3H]oxytocin with a much lower affinity; these sites were not labelled in the presence of 5 nM [3H]oxytocin, and 10 microM oxytocin displaced [3H]vasopressin binding by 80%. Such sites occurred in several thalamic nuclei, in the dopaminergic A13 cell group of the zona incerta, the suprachiasmatic nucleus, the fundus striati and the lateral septal nucleus. No selective oxytocin sites were detected. Different oxytocin and vasopressin binding characteristics were found in the hypothalamo-neurohypophysial system. In the paraventricular and supraoptic nuclei and in the pituitary neural lobe the [3H]vasopressin binding density was twice that of [3H]oxytocin; vasopressin was always more potent than oxytocin in displacing both [3H]vasopressin and [3H]oxytocin binding from those sites. Interaction of the tritiated peptides with neurophysins cannot be completely ruled out in these locations. The present data are discussed in correlation with the functional roles of the neurohypophysial peptides in the brain and the pharmacological characteristics of their receptors.

Vasopressin stimulates inositol phospholipid metabolism in rat medulla oblongata in vivo

We have investigated whether microinjections of arginine-vasopressin (AVP) into the nucleus tractus solitarii (NTS) of rats results in the stimulation of inositol phospholipid metabolism, a biochemical event related to the activation of V1-type receptors. We found that AVP, but not the related peptides oxytocin or deamino-D-AVP, increases the incorporation of [3H]myoinositol into lipids and the accumulation of labelled inositol phosphates in the dorsomedial medulla oblongata of lithium-treated rats. These effects were abolished by local pretreatment with D-(CH2)5-Tyr(Me)-AVP, a specific antagonist of V1-receptors. It is concluded that the effects of AVP in the NTS of rats are likely to be due to an action on specific receptors (V1-like) and subsequent activation of polyphosphoinositide turnover.

125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT: a selective oxytocin receptor ligand

An oxytocic antagonist, [1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid,2-O-methyltyrosine,4-threonine, 8-ornithine,9-tyrosylamide]vasotocin (d(CH2)5[Tyr(Me)2, Thr4,Tyr-NH2(9)]OVT [corrected], was monoiodinated at the phenyl moiety of the tyrosylamide residue at position 9. 125I-labelling was performed with 1,3,4,6-tetrachloro-3 alpha,6 alpha-diphenyl-glycoluril. Iodination resulted in an increased affinity for rat uterine oxytocin receptors. A considerably lower affinity for rat vascular V1- and renal V2-receptors was found, resulting in a highly specific oxytocin receptor ligand. 125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT [corrected] was demonstrated to bind selectively to one population of binding sites in rat uterus and ventral hippocampal membrane preparations. Dissociation constants ranged between 0.03 and 0.06 nM. After 3 days of exposure autoradiography revealed binding in regions known to contain oxytocin receptors as well as labelling in some new regions, while no binding was found in the lateral septum, a structure containing mainly [8-arginine]vasopressin receptors. The high specific radioactivity of 125I-labelling allowed important reductions in membrane protein amount, gain in precision of binding analysis as well as considerably lower exposure times for autoradiography.

Localization and pharmacological characterization of high affinity binding sites for vasopressin and oxytocin in the rat brain by light microscopic autoradiography

Sites which bind tritiated vasopressin (AVP) with high affinity were detected in the brain of male, adult rats, by light microscopic autoradiography. Their anatomical localization differed markedly from that of high affinity binding sites for tritiated oxytocin (OT) determined in the same animal. Co-labelling was minimized by using low concentrations of [3H]AVP and [3H]OT. Binding of the former occurred predominantly in several structures of the limbic system (septum, amygdala, bed nucleus of the stria terminalis, accumbens nucleus), in two hypothalamic nuclei (suprachiasmatic and dorsal tuber) and in the area of the nucleus of the solitary tract. Binding of OT was evidenced in the olfactory tubercle, the ventromedial hypothalamic nucleus, the central amygdaloid nucleus and the ventral hippocampus. The ligand specificity of the binding sites was assessed in competition experiments. Synthetic structural analogues were used, allowing to discriminate OT receptors (OH[Thr4,Gly7]OT) from V2 receptors (dDAVP and d[Tyr(Me)2]VDAVP), V1 receptors ([Phe2,Orn8]VT) and V1b receptors (desGly9d(CH2)5AVP). Our main conclusions are, firstly, that AVP and OT binding sites can be readily distinguished, and that there is virtually no overlap in their distribution in the rat brain. Second, we showed that the sites which bind AVP with high affinity in the brain are V1 receptors, different both from the renal V2 receptors and from the anterior pituitary V1b receptors. Our results support the conjecture that AVP and OT play a role in interneuronal communication in the brain.

Regulatory peptide receptors: visualization by autoradiography

The receptors for regulatory peptides have been extensively characterized using radioligand binding techniques. By combining these binding techniques with autoradiography it is possible to visualize at the light and electron microscopic levels the anatomical and cellular localization of these receptors. In this review we discuss the procedures used to label peptide receptors for autoradiography and the peculiarities of peptides as ligands. The utilization of autoradiography in mapping peptide receptors in brain and peripheral tissues, some of the new insights revealed by these studies particularly the problem of 'mismatch' between endogenous peptides and receptors, the existence of multiple receptors for a given peptide family and the use of peptide receptor autoradiography in human tissues are also reviewed.

Facilitatory effect of oxytocin on oxytocin cell background activity in the rat is suckling-dependent

The injection of oxytocin into the third ventricle during suckling in lactating rats not only enhances neurosecretory bursts but also the background activity of oxytocin cells. However, removing the young rats 10 min after the oxytocin injection is immediately followed by a decrease in background activity, while injecting oxytocin alone (i.e. without suckling) or suckling alone (i.e. without oxytocin injection) has no effect. These results, which show that the facilitatory effect of oxytocin on oxytocin cell background activity is suckling-dependent, suggest that oxytocin could act either on the afferent pathways for milk ejection or on oxytocin cells themselves, but only if they received inputs from mammary glands. Various hypotheses on the site and mode of action of oxytocin are discussed.