Oxytocin-responsive cells in the mammalian nervous system

Stimulation of the hypothalamic paraventricular nucleus modulates cardiorespiratory responses via oxytocinergic innervation of neurons in pre-Botzinger complex

Previously we reported that oxytocin (OT)-containing neurons of the hypothalamic paraventricular nucleus (PVN) project to the pre-Bötzinger complex (pre-BötC) region and phrenic motoneurons innervating the diaphragm (D). The aim of these studies was to determine pathways involved in PVN stimulation-induced changes in upper airway and chest wall pumping muscle activity. In addition, we determined the role of OT-containing neurons in the PVN in mediating increased respiratory output elicited by PVN stimulation. Neuroanatomical experiments, using pseudorabies virus (PRV) as a transneuronal tracer in C8 spinalectomized animals showed that PVN neurons project to hypoglossal motoneurons innervating the genioglossus (GG) muscle. Furthermore, microinjection of the PVN with bicuculline, a GABA(A) receptor antagonist, significantly increased (P < 0.05) peak electromyographic activity of GG (GG(EMG)) and of D(EMG), frequency discharge, and arterial blood pressure (BP) and heart rate. Prior injection of OT antagonist [d-(CH(2))(5),Tyr(Me)(2),Orn(8)]-vasotocin intracisternally or blockade of OT receptors in the pre-BötC region with OT antagonist l-368,899, diminished GG(EMG) and D(EMG) responses and blunted the increase in BP and heart rate to PVN stimulation. These data show that PVN stimulation affects central regulatory mechanisms via the pre-BötC region controlling both respiratory and cardiovascular functions. The parallel changes induced by PVN stimulation were mediated mainly through an OT-OT receptor signaling pathway.

Vasopressin and oxytocin receptor mRNA expression during rat telencephalon development

We investigated the developmental expression of vasopressin and oxytocin receptor and peptide mRNA using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot hybridization. Messenger RNAs for both vasopressin receptor subtypes V(1)a and V(2)were present in the telencephalon from embryonic day 12 to day 20. Both V(1)a and V(2)receptor mRNA increased on day 13 and then remained stable from embryonic day 13 to day 20. Messenger RNA for the vasopressin peptide was also detected in the telencephalon from day 12 to day 20, indicating that vasopressin could be synthesized within the rat cerebral cortex during rat embryonic development. Oxytocin receptor mRNA expression was also present in the telencephalon, but expression levels varied considerably from day 12 to day 20. No oxytocin mRNA expression was detected during rat telencephalon development. Temporal patterns of vasopressin receptor and vasopressin peptide mRNA expression along with oxytocin receptor mRNA suggest a temporal role for vasopressin- and oxytocin-mediated actions during rat telencephalon development.

Vasopressin and oxytocin action in the brain: cellular neurophysiological studies

During the last two decades it has become apparent that vasopressin (VP) and oxytocin (OT), in addition to playing a role as peptide hormones, also act as neurotransmitters. Morphological studies and electrophysiological recordings have shown a close anatomical correlation between the presence of these receptors and the neuronal responsiveness to VP or OT. These compounds have been found to affect membrane excitability in neurons located in the hippocampus, hypothalamus, lateral septum, brainstem, spinal cord and superior cervical ganglion. Sharp electrode intracellular and whole-cell recordings, done in brainstem motoneurons, have revealed that VP and OT can directly affect neuronal excitability by opening non-specific cationic channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas in some hypothalamic neurons OT appears to mobilize intracellular calcium, as revealed by calcium imaging techniques, in the brainstem the action of this neuropeptide is mediated by a second messenger which is distinct from the second messenger activated in peripheral target cells. Future studies should be aimed at elucidating the properties of the cationic channels responsible for the neuronal action of VP and OT, at identifying the brain-specific second messengers activated by these neuropeptides and at determining whether endogenous VP and OT can exert neuronal effects similar to those elicited by exogenous neuropeptides.

Distribution of vasopressin and oxytocin receptors in the rat spinal cord: sex-related differences and effect of castration in pudendal motor nuclei

The distribution of vasopressin and oxytocin receptors was established by in vitro autoradiography in the spinal cord of adult rats of either sex, as well as in male castrates. In both males and females, high concentrations of vasopressin binding sites were found in a few groups of somatic motoneurons: the large lateral group at the cervicothoracic junction in segments C8 and Th1; the small medial group in segments L3-L5; and the pudendal and retrodorsolateral nuclei in segments L5-L6. The extension and intensity of labelling in pudendal nuclei were markedly lower in females than in males, in particular in the dorsomedial nucleus, where binding was either not or hardly detectable. Gonadectomy in males resulted in a significant reduction of binding in pudendal nuclei, but not in other labelled motor nuclei. Moderate amounts of vasopressin binding sites were also found evenly distributed throughout the central gray at all segmental levels. Oxytocin binding sites were detectable in all spinal segments, but in low amounts and restricted to the superficial layers of the dorsal horn. The abundance of vasopressin binding sites in the central gray suggests that vasopressin may be involved in most spinal functions. The permanent expression of vasopressin binding sites in pudendal motor nuclei of is particular interest with regard to the known plasticity of pudendal motoneurons.

Changes in hypothalamic oxytocin levels during morphine tolerance

The role of hypothalamic oxytocin neurons in the hypothalamus-pituitary-adrenal (HPA) axis adaptation during opioid tolerance has not been explored. In this study the modification of oxytocin levels in different hypothalamic nuclei was determined after acute or chronic morphine exposure. Male rats were implanted with placebo (naïve) or morphine (tolerant) pellets for 7 days. On day 8, groups of rats received an acute injection of either saline i.p. or morphine (30 mg/kg i.p.) and were sacrificed 30 min later. In morphine-tolerant rats, there was a decrease in the oxytocin content in the median eminence (ME) and in the supraoptic nucleus (SO) after acute injection of saline or morphine. No modifications were seen in the paraventricular nucleus (PVN). The present study demonstrates that chronic morphine administration alters the brain oxytocin system, which suggests that this peptide might contribute to the behavioural, emotional and neuroendocrine responses to opioids.

Oxytocin and vasopressin mRNA expression in rat hypothalamus following kainic acid-induced seizures

In this study, the regulation of hypothalamic oxytocin and vasopressin messenger RNA expression following the induction of seizures was investigated by in situ hybridization. Following kainic acid-induced seizures, a significant increase in oxytocin messenger RNA in the paraventricular nucleus was demonstrated at 1.5 h, one and two weeks; its level decreased at three weeks and was significantly increased again at four weeks; at eight weeks the messenger RNA level still remained higher than that of controls. Vasopressin messenger RNA in the paraventricular nucleus was increased significantly only at 1.5 h following induction of seizures. The oxytocin messenger RNA level in the supraoptic nucleus was also increased early at 1.5 h and later at four weeks following seizures; however, these increases did not last as long as those in the paraventricular nucleus. Vasopressin messenger RNA in the supraoptic nucleus was also increased after the initial seizures; however, its messenger RNA level vacillated up and down throughout the post-seizure times studied. The earliest significant increase of vasopressin messenger RNA was at one week after seizures, and there was a late significant increase of vasopressin messenger RNA at three weeks after seizures. The present study demonstrates that following kainic acid-induced seizures both, the oxytocin and vasopressin messenger RNA expressions, were up-regulated and these up-regulations were long-term events. The increase of oxytocin messenger RNA in the paraventricular nucleus was more persistent than the others. The pattern of messenger RNA up-regulation was different for oxytocin and vasopressin, and different in the paraventricular nucleus and supraoptic nucleus. These different patterns of messenger RNA elevations suggest that the different components of the rat hypothalamus were regulated differentially by kainic acid-induced seizures.