Comparative study of dopamine- and noradrenaline-immunoreactive terminals in the paraventricular and supraoptic nuclei of the rat

Convergence of oxytocin and dopamine signalling in neuronal circuits: Insights into the neurobiology of social interactions across species

Social behaviour is essential for animal survival, and the hypothalamic neuropeptide oxytocin (OXT) critically impacts bonding, parenting, and decision-making. Dopamine (DA), is released by ventral tegmental area (VTA) dopaminergic neurons, regulating social cues in the mesolimbic system. Despite extensive exploration of OXT and DA roles in social behaviour independently, limited studies investigate their interplay. This narrative review integrates insights from human and animal studies, particularly rodents, emphasising recent research on pharmacological manipulations of OXT or DA systems in social behaviour. Additionally, we review studies correlating social behaviour with blood/cerebral OXT and DA levels. Behavioural facets include sociability, cooperation, pair bonding and parental care. In addition, we provide insights into OXT-DA interplay in animal models of social stress, autism, and schizophrenia. Emphasis is placed on the complex relationship between the OXT and DA systems and their collective influence on social behaviour across physiological and pathological conditions. Understanding OXT and DA imbalance is fundamental for unravelling the neurobiological underpinnings of social interaction and reward processing deficits observed in psychiatric conditions.

Windows into stress: a glimpse at emerging roles for CRHPVN neurons

The corticotropin-releasing hormone cells in the paraventricular nucleus of the hypothalamus (CRH^PVN) control the slow endocrine response to stress. The synapses on these cells are exquisitely sensitive to acute stress, leveraging local signals to leave a lasting imprint on this system. Additionally, recent work indicates that these cells also play key roles in the control of distinct stress and survival behaviors. Here we review these observations and provide a perspective on the role of CRH^PVN neurons as integrative and malleable hubs for behavioral, physiological, and endocrine responses to stress.

Neurobiology of Loneliness, Isolation, and Loss: Integrating Human and Animal Perspectives

In social species such as humans, non-human primates, and even many rodent species, social interaction and the maintenance of social bonds are necessary for mental and physical health and wellbeing. In humans, perceived isolation, or loneliness, is not only characterized by physical isolation from peers or loved ones, but also involves negative perceptions about social interactions and connectedness that reinforce the feelings of isolation and anxiety. As a complex behavioral state, it is no surprise that loneliness and isolation are associated with dysfunction within the ventral striatum and the limbic system - brain regions that regulate motivation and stress responsiveness, respectively. Accompanying these neural changes are physiological symptoms such as increased plasma and urinary cortisol levels and an increase in stress responsivity. Although studies using animal models are not perfectly analogous to the uniquely human state of loneliness, studies on the effects of social isolation in animals have observed similar physiological symptoms such as increased corticosterone, the rodent analog to human cortisol, and also display altered motivation, increased stress responsiveness, and dysregulation of the mesocortical dopamine and limbic systems. This review will discuss behavioral and neuropsychological components of loneliness in humans, social isolation in rodent models, and the neurochemical regulators of these behavioral phenotypes with a neuroanatomical focus on the corticostriatal and limbic systems. We will also discuss social loss as a unique form of social isolation, and the consequences of bond disruption on stress-related behavior and neurophysiology.

Disturbance of the reciprocal-interaction between the OXTergic and DAergic systems in the CNS causes atypical social behavior in syntaxin 1A knockout mice

Several studies have shown that oxytocin (OXT) modulates social behavior. Similarly, monoamines such as dopamine (DA) play a role in regulating social behavior. Previous studies have demonstrated that the soluble N-ethylmaleimide-sensitive fusion attachment protein receptor (SNARE) protein syntaxin 1A (STX1A) regulates the secretion of OXT and monoamines, and that STX1A gene knockout (STX1A KO) mice exhibit atypical social behavior, such as deficient social recognition, due to reduced OXT release. In this study, we analyzed the neural mechanism regulating social behavior by OXT and/or DA using STX1A KO mice as a model animal. We found that OXT directly induced DA release from cultured DA neurons through OXT and V1a receptors. In STX1A KO mice, the atypical social behavior was partially improved by OXT administration, which was inhibited by D1 receptor blockade. In addition, the atypical social behavior in STX1A KO mice was partially improved by facilitation of DAergic signaling with the DA reuptake inhibitor GBR12909. Moreover, the amelioration by GBR12909 was inhibited by OXTR blockade. These results suggest that the reciprocal interaction between the DAergic and OXTergic neuronal systems in the CNS may be important in regulating social behavior.

Role of central oxytocin and dopamine systems in nociception and their possible interactions: suggested hypotheses

Central oxytocin and dopamine have an important role in the process of nociception at the spinal level as well as supraspinal structures, e.g. anterior cingulate cortex, insular cortex, amygdala, nucleus accumbens, and hypothalamus. Many studies have pointed out the importance of both systems in the pain descending modulatory system and in pain-related symptoms in some chronic disorders, e.g. Parkinson disease and fibromyalgia. The interaction between oxytocin and dopamine systems has been addressed in some motivational behaviors, e.g. maternal and sexual behaviors, pair bonding, and salience. In this aspect, we propose that an oxytocin-dopamine interaction could be present in nociception, and we also explain the possible hypotheses of such an interaction between these systems.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

Oxytocin and social motivation

Humans are fundamentally social creatures who are ‘motivated’ to be with others. In this review we examine the role of oxytocin (OT) as it relates to social motivation. OT is synthesized in the brain and throughout the body, including in the heart, thymus, gastrointestinal tract, as well as reproductive organs. The distribution of the OT receptor (OTR) system in both the brain and periphery is even more far-reaching and its expression is subject to changes over the course of development. OTR expression is also sensitive to changes in the external environment and the internal somatic world. The OT system functions as an important element within a complex, developmentally sensitive biobehavioral system. Other elements include sensory inputs, the salience, reward, and threat detection pathways, the hypothalamic-pituitary-gonadal axis, and the hypothalamic-pituitary-adrenal stress response axis. Despite an ever expanding scientific literature, key unresolved questions remain concerning the interplay of the central and peripheral components of this complex biobehavioral system that dynamically engages the brain and the body as humans interact with social partners over the course of development.

Dopamine and oxytocin interactions underlying behaviors: potential contributions to behavioral disorders

Dopamine is an important neuromodulator that exerts widespread effects on the central nervous system (CNS) function. Disruption in dopaminergic neurotransmission can have profound effects on mood and behavior and as such is known to be implicated in various neuropsychiatric behavioral disorders including autism and depression. The subsequent effects on other neurocircuitries due to dysregulated dopamine function have yet to be fully explored. Due to the marked social deficits observed in psychiatric patients, the neuropeptide, oxytocin is emerging as one particular neural substrate that may be influenced by the altered dopamine levels subserving neuropathologic-related behavioral diseases. Oxytocin has a substantial role in social attachment, affiliation and sexual behavior. More recently, it has emerged that disturbances in peripheral and central oxytocin levels have been detected in some patients with dopamine-dependent disorders. Thus, oxytocin is proposed to be a key neural substrate that interacts with central dopamine systems. In addition to psychosocial improvement, oxytocin has recently been implicated in mediating mesolimbic dopamine pathways during drug addiction and withdrawal. This bi-directional role of dopamine has also been implicated during some components of sexual behavior. This review will discuss evidence for the existence dopamine/oxytocin positive interaction in social behavioral paradigms and associated disorders such as sexual dysfunction, autism, addiction, anorexia/bulimia, and depression. Preliminary findings suggest that whilst further rigorous testing has to be conducted to establish a dopamine/oxytocin link in human disorders, animal models seem to indicate the existence of broad and integrated brain circuits where dopamine and oxytocin interactions at least in part mediate socio-affiliative behaviors. A profound disruption to these pathways is likely to underpin associated behavioral disorders. Central oxytocin pathways may serve as a potential therapeutic target to improve mood and socio-affiliative behaviors in patients with profound social deficits and/or drug addiction.

Tachykinin NK3 receptor contribution to systemic release of vasopressin and oxytocin in response to osmotic and hypotensive challenge

Activation of the neurokinin 3 receptor (NK3R) by a receptor agonist, hypotension, and hyperosmolarity results in the internalization of NK3R expressed by magnocellular neurons and the release of vasopressin (VP) and oxytocin (OT) into the circulation. The contribution of NK3R activation to the release of VP and OT in response to hyperosmolarity and hypotension was evaluated by measuring the release of both hormones following pretreatment with a selective NK3R antagonist, SB-222200. Freely behaving male rats were given an intraventricular injection of either 0.15 M NaCl or 250, 500, or 1,000 pmol SB-222200, and then were administered an intravenous infusion of 2 M NaCl or 0.15 M NaCl (experiment 1), or a bolus intra injection of 0.15 M NaCl or hydralazine (HDZ), a hypotension-inducing drug (experiment 2). Blood samples were taken from indwelling arterial catheters at various time points for 1-2 h, both before and after treatments. Plasma VP and OT levels were determined by ELISA. Blockade of NK3R did not affect the baseline levels of either hormone. In contrast, pretreatment with SB-222200 significantly reduced ( approximately 60%) or abolished the release of VP and OT, respectively, to 2 M NaCl infusion. HDZ-induced VP and OT release was eliminated by pretreatment with 500 pmol SB-222200. Therefore, NK3R activation contributes significantly to the systemic release of both VP and OT in response to osmotic and hypotensive challenges.

Noradrenergic axon terminals contact gastric preautonomic neurons in the paraventricular nucleus of the hypothalamus in rats

Hypothalamic neural activity is modulated by viscerosensory signals that are carried in large part by noradrenergic (NA) inputs to the paraventricular nucleus of the hypothalamus (PVN). The present study examined the ultrastructural relationship of NA axon varicosities with the somata and dendrites of identified gastric preautonomic PVN neurons in adult male rats. NA varicosities were visualized by immunoperoxidase labeling of dopamine beta hydroxylase (DbH), and gastric preautonomic PVN neurons were identified by immunogold labeling of pseudorabies virus (PRV) transported retrogradely and transneuronally from injection sites in the stomach wall. Among 1,136 DbH-positive varicosities identified within the parvocellular PVN in four rats, approximately 36% formed either a close apposition or a synaptic contact with a somatic or dendritic profile. The majority of identified contacts between DbH- and PRV-positive profiles were classified as close appositions that lacked clear synaptic specializations. Approximately 65% of identified synaptic contacts between DbH- and PRV-positive profiles were classified as symmetric (Gray's type II) synapses. DbH-positive terminals formed close appositions and synaptic contacts with dendritic and somatic compartments of PRV-positive neurons, although dendrites were contacted nearly five times more often than somata. These findings invite continued work to delineate the functional role of NA signaling pathways in conveying interoceptive signals to preautonomic PVN neurons under normal and pathophysiological conditions.

Visceral sensory inputs to the endocrine hypothalamus

Interoceptive feedback signals from the body are transmitted to hypothalamic neurons that control pituitary hormone release. This review article describes the organization of central neural pathways that convey ascending visceral sensory signals to endocrine neurons in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus in rats. A special emphasis is placed on viscerosensory inputs to corticotropin releasing factor (CRF)-containing PVN neurons that drive the hypothalamic-pituitary-adrenal axis, and on inputs to magnocellular PVN and SON neurons that release vasopressin (AVP) or oxytocin (OT) from the posterior pituitary. The postnatal development of these ascending pathways also is considered.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

Dopamine D4 receptor-mediated presynaptic inhibition of GABAergic transmission in the rat supraoptic nucleus

The mechanism by which dopamine induces or facilitates neurohypophysial hormone release is not completely understood. Because oxytocin- and vasopressin-secreting supraoptic neurons are under the control of a prominent GABAergic inhibition, we investigated the possibility that dopamine exerts its action by modulating GABA-mediated transmission. Whole cell voltage-clamp recordings of supraoptic neurons were carried out in acute hypothalamic slices to determine the action of dopamine on inhibitory postsynaptic currents. Application of dopamine caused a consistent and reversible reduction in the frequency, but not the amplitude, of miniature synaptic events, indicating that dopamine was acting presynaptically to reduce GABAergic transmission. The subtype of dopamine receptor involved in this response was characterized pharmacologically. Dopamine inhibitory action was greatly reduced by two highly selective D4 receptor antagonists L745,870 and L750,667 and to a lower extent by the antipsychotic drug clozapine but was unaffected by SCH 23390 and sulpiride, D1/D5 and D2/D3 receptor antagonists, respectively. In agreement with these results, the action of dopamine was mimicked by the potent D4 receptor agonist PD168077 but not by SKF81297 and bromocriptine, D1/D5 and D2/D3 receptor agonists, respectively. Dopamine and PD168077 also reduced the amplitude of evoked inhibitory postsynaptic currents, an effect that was accompanied by an increase in paired-pulse facilitation. These data clearly indicate that D4 receptors are located on GABA terminals in the supraoptic nucleus and that their activation reduces GABA release in the supraoptic nucleus. Therefore dopaminergic facilitation of neurohypophysial hormone release appears to result, at least in part, from disinhibition of magnocellular neurons caused by the depression of GABAergic transmission.

Maternity leads to morphological synaptic plasticity in the oxytocin system

The oxytocinergic system, which plays a major role in the control of different aspects of maternity, undergoes extensive synaptic and neuronal-glial remodelling during parturition and lactation and has thus become a remarkable example of activity-dependent morphological synaptic plasticity in the adult mammalian brain. The use of different comparative ultrastructural analyses on the rat supraoptic and paraventricular nuclei, together with identification of pre- and post-synaptic elements, has allowed us to show that there is a significant increase in the number of GABAergic, glutamatergic and noradrenergic synapses impinging on oxytocin neurons, concomitant with a reduction of glial coverage of the neurons. This synaptic plasticity involves axo-dendritic and axo-somatic contacts originating from terminals making one or several synaptic contacts in one plane of section. While noradrenergic afferents arise from medullary catecholaminergic neurons, our recent in vitro observations indicate that GABAergic and glutamatergic afferents derive, at least partly, from local intrahypothalamic neurons, in close proximity to oxytocin neurons. The cellular mechanisms underlying this morphological synaptic plasticity remain to be determined but it is highly likely that they depend on increased activity in both pre- and post-synaptic elements. Moreover, the oxytocin system continues to express 'embryonic' molecular features that may allow the morphological plasticity to occur. In particular, it expresses high levels of cell surface adhesion molecules currently thought to intervene in synaptic remodelling in the developing and lesioned central nervous system, including the weakly adhesive polysialylated isoform of the Neural Cell Adhesion Molecule, the axonal glycoprotein F3 and its ligand, the extracellular matrix glycoprotein, tenascin-C.

Dopamine D4 receptor activation inhibits presynaptically glutamatergic neurotransmission in the rat supraoptic nucleus

Oxytocin and vasopressin release from magnocellular neurons of the supraoptic nucleus is under the control of glutamate-dependent excitation. The supraoptic nucleus also receives a generalized dopaminergic input from hypothalamic sources. To determine if dopamine can influence this excitatory drive onto the magnocellular neurons, we used whole-cell patch clamp to record the effect of dopamine on evoked and miniature excitatory postsynaptic currents in rat hypothalamic slices. Dopamine exposure (30 microM to 1 mM) induced a large and reversible reduction in the amplitude of evoked excitatory postsynaptic current in nearly all magnocellular cells tested. D4 receptors appeared to mediate dopamine's activity, based on inhibition of the response with 50 microM clozapine, but not by SCH 23390 or sulpiride, and mimicry of dopamine's action with the D4 specific agonist, PD 168077. Analysis of paired-pulse experiments and miniature postsynaptic currents indicated that dopamine's action involved a presynaptic mechanism, since the frequency of miniature postsynaptic currents was reduced with dopamine exposure without any change in current kinetics or amplitude, while the paired-pulse ratio increased. We therefore have demonstrated for the first time a role for dopamine D4 receptors in the supraoptic nucleus in the presynaptic inhibition of glutamatergic neurotransmission onto magnocellular neurons.

Dopamine D(5) receptor immunolocalization in rat and monkey brain

Dopamine D(5) receptor localization has been difficult because even the most specific ligands cannot distinguish between molecular subtypes of the D(1)-like receptor subfamily. Antifusion protein rabbit polyclonal antibodies directed against the C-terminus of human D(5) receptor were therefore developed for immunolocalization of the D(5) receptor protein in brain. The antibodies were characterized by immunoblot analysis and immunoprecipitation and used for light microscopic immunocytochemistry in rat and monkey brain. Affinity purified D(5) antibodies were specific for D(5) fusion protein as well as cloned and native D(5) receptor on Western blots, and D(5) antisera specifically immunoprecipitated solubilized, cloned D(5) receptor. Regional distribution of D(5) receptor immunoreactivity was consistent across species and correlated well with D(5) mRNA distribution previously reported in monkey brain. Immunoreactivity was widespread and tended to label perikarya and proximal dendrites of neurons in cerebral cortex, basal ganglia, basal forebrain, hippocampus, diencephalon, brainstem, and cerebellum. Neuropil was immunoreactive in olfactory bulb, islands of Calleja, cerebral cortex, superior colliculus, and molecular layer of cerebellum. The distribution of D(5) in brain was clearly different from that of other dopamine receptor subtypes, including D(1), the other member of the D(1)-like receptor subfamily. This unique distribution corroborates the idea that the D(5) receptor subtype has a distinct role in dopamine neurotransmission.

Neuronal projections to the lateral retrochiasmatic area of sheep with special reference to catecholaminergic afferents: immunohistochemical and retrograde tract-tracing studies

The retrochiasmatic area contains the A15 catecholaminergic group and numerous monoaminergic afferents whose discrete cell origins are unknown in sheep. Using tract-tracing methods with a specific retrograde fluorescent tracer, fluorogold, we examined the cells of origin of afferents to the retrochiasmatic area in sheep. The retrogradely labeled cells were seen by observation of the tracer by direct fluorescence or by immunohistochemistry with specific antibodies raised in rabbits or horses. Among the retrogradely labeled neurons, double immunohistochemistry for tyrosine hydroxylase, dopamine-beta-hydroxylase, and serotonin were used to characterize catecholamine and serotonin FG labeled neurons. The retrochiasmatic area, which included the A15 dopaminergic group and the accessory supraoptic nucleus (SON), received major inputs from the lateral septum (LS), the bed nucleus of the stria terminalis (BNST), the thalamic paraventricular nucleus, hypothalamic paraventricular and supraoptic nuclei, the perimamillary area, the amygdala, the ventral part of the hippocampus and the parabrachial nucleus (PBN). Further, numerous scattered retrogradely labeled neurons were observed in the preoptic area, the ventromedial part of the hypothalamus. the periventricular area, the periaqueductal central gray (CG), the ventrolateral medulla and the dorsal vagal complex. Most of the noradrenergic afferents came from the ventro-lateral medulla (Al group), and only a few from the locus coeruleus complex (A6/A7 groups). A few dopaminergic neurons retrogradely labeled with flurogold were observed in the periventricular area of the hypothalamus. Rare serotoninergic fluorogold labeled neurons belonged to the dorsal raphe nucleus. Most of these afferents came from both sides of the brain, except for hypothalamic supraoptic and paraventricular nuclei. In the light of these anatomical data, we compared our results with data obtained from rats, and we discussed the putative role of these afferents in sheep in the regulation of several specific functions in which the retrochiasmatic area may be involved, such as reproduction.

Activity-dependent morphological synaptic plasticity in an adult neurosecretory system: magnocellular oxytocin neurons of the hypothalamus

Oxytocin and vasopressin neurons, located in the supraoptic and paraventricular nuclei of the hypothalamus, send their axons to the neurohypophysis where the neurohormones are released directly into the general circulation. Hormone release depends on the electrical activity of the neurons, which in turn is regulated by different afferent inputs. During conditions that enhance oxytocin secretion (parturition, lactation, and dehydration), these afferents undergo morphological remodelling which results in an increased number of synapses contacting oxytocin neurons. The synaptic changes are reversible with cessation of stimulation. Using quantitative analyses on immunolabelled preparations, we have established that this morphological synaptic plasticity affects both inhibitory and excitatory afferent inputs to oxytocin neurons. This review describes such synaptic modifications, their functional significance, and the cellular mechanisms that may be responsible.Key words: oxytocin, vasopressin, GABA, glutamate, noradrenaline, hypothalamo-neurohypophysial system, lactation.

Hypothalamic A14 and A15 catecholamine cells provide the dopaminergic innervation to the supraoptic nucleus in rat: a combined retrograde tracer and immunohistochemical study

This study investigated the origin of a dopaminergic innervation of the hypothalamic supraoptic nucleus. In pentobarbital-anaesthetized male Long-Evans rats, a transpharyngeal approach was used to inject a retrograde tracer, rhodamine latex microspheres, into the supraoptic nucleus. After 13-26 h survival under anaesthesia, animals were perfused transcardially, the brain sectioned and processed for tyrosine hydroxylase immunofluorescence, a marker for hypothalamic dopaminergic neurons. In six cases with injections restricted to the supraoptic nucleus, rhodamine-labelled microspheres were observed in a population of tyrosine hydroxylase-positive neurons located in the A15 cells below the anterior commissure (A15 dorsal) and above the optic chiasm (A15 ventral), and the dorsal and lateral periventricular A14 cell group. Occasional double-labelled cells were seen in the medial and lateral hypothalamus and bed nucleus of the stria terminalis, but rarely in other known dopaminergic cell groups, notably the ventral tegmental area (A10), zona incerta (A13) and substantia nigra. In support of a role for dopamine in neurohypophysial regulation, these observations indicate that the major dopaminergic input to magnocellular neurons in the hypothalamic supraoptic nucleus is derived from a relatively sparse population of neurons located in the A14 and A15 cell groups.

Visualization of local afferent inputs to magnocellular oxytocin neurons in vitro

We recently showed that oxytocin (OT) neurons in organotypic slice cultures obtained from postnatal rat hypothalamus display complex patterns of electrical activity, similar to those of adult magnocellular OT neurons in vivo. Here we used such cultures to investigate the identity and, in particular, the origin of afferent inputs responsible for this activity. Multiple immunostaining with light and confocal microscopy showed that the somata and dendrites of oxytocinergic neurons were contacted by numerous synapses, visualized by their reaction to the synaptic markers, synaptophysin or synapsin. Many were GABAergic, displaying immunoreactivities for glutamic acid decarboxylase or gamma-aminobutyric acid (GABA); others were enriched in glutamate immunoreactivity. Such afferents presumably arose from GABA- or glutamate-immunoreactive neurons, respectively, with distinct and characteristic morphologies and topographies. A few dopaminergic boutons (tyrosine hydroxylase- or dopamine-immunopositive) impinged on OT neurons; they arose from dopamine-positive neurons located along the third ventricle. No noradrenergic profiles were detected. Despite the presence of choline acetyl-transferase (ChAT)-immunoreactive neurons, there were no cholinergic contacts. Lastly, we found oxytocinergic synapses, identified by immunoreaction for OT-related neurophysin and synapsin, contacting OT somata and dendrites. Our observations thus demonstrate that inhibitory and excitatory inputs to OT neurons derive from local intrahypothalamic GABA and glutamate neurons, in close proximity to the neurons. They also reveal that OT neurons are innervated by hypothalamic dopaminergic neurons. Finally, they confirm the existence of homotypic OT synaptic contacts which derive from local OT neurons.

Stimulation of central and systemic oxytocin release by histamine in the paraventricular hypothalamic nucleus: evidence for an interaction with norepinephrine

Central histaminergic neurons have been implicated in the control of oxytocin (OT) secretion in various physiological conditions, including parturition and lactation. The present studies investigated whether histamine also influences the central intranuclear release of OT, which is known to be important in the activation of OT neurons, and the possible interaction of histamine with norepinephrine in systemic and central OT release. Microdialysis probes were placed immediately adjacent to the hypothalamic paraventricular nucleus (PVN) and used for administration of artificial cerebrospinal fluid (ACSF) vehicle, ACSF containing histamine, ACSF containing histamine in combination with a specific H1 or H2 histamine receptor antagonist, or ACSF containing histamine and the alpha-adrenergic antagonist phentolamine. Dialysates and plasma were collected, and OT concentrations were determined using RIA. Dialysis of the PVN with ACSF containing histamine significantly increased the release of OT systemically and centrally within the PVN. Furthermore, the increases in OT concentration in dialysates and plasma were prevented by simultaneous administration of chlorpheniramine (an H1 receptor antagonist) or ranitidine (an H2 receptor antagonist) as well as by the adrenergic antagonist phentolamine. These data demonstrate that histamine acts within the PVN to increase both systemic and intranuclear release of OT. Furthermore, the increased OT release induced by histamine is dependent upon stimulation of both H1 and H2 histaminergic receptors and subsequent activation of alpha-noradrenergic receptors. These findings suggest that histamine induces systemic and intranuclear OT release by stimulating the release of norepinephrine.

Dopaminergic control of angiotensin II-induced vasopressin secretion in vitro

Because dopamine influences arginine vasopressin (AVP) release, the present studies were designed to ascertain the dopamine receptor subtype that potentiates angiotensin II-induced AVP secretion in cultured hypothalamo-neurohypophysial explants. Dopamine (a nonselective D1/D2 agonist), apomorphine (a D2 >> D1 agonist), and SKF-38393 (a selective D1 agonist) dose dependently increased AVP secretion. Maximal AVP release was observed with 5 microM dopamine, 307 +/- 66% . explant-1 . h-1, 1 microM SKF-38393, 369 +/- 41% . explant-1 . h-1, and 0.1 microM apomorphine, 374 +/- 67% . explant-1 . h-1. Selective D1 antagonism with 1 microM SCH-23390 blocked AVP secretion to values no different from basal. Domperidone (D2 antagonist), phenoxybenzamine (nonselective adrenergic antagonist), and prazosin (alpha1-antagonist) failed to prevent release. D1 antagonism also prevented AVP secretion to 1 microM angiotensin II [angiotensin II, 422 +/- 87% . explant-1 . h-1 vs. angiotensin II plus SCH-23390, 169 +/- 28% . explant-1 . h-1 (P < 0.05)], but D2 and alpha1-adrenergic blockade did not. In contrast, AT1 receptor inhibition with 0.5 microM losartan blocked angiotensin II- but not dopamine-induced AVP release. AT2 antagonism had no effect. Although subthreshold doses of the agonists did not increase AVP secretion (0. 05 microM dopamine, 133 +/- 44% . explant-1 . h-1; 0.01 microM SKF-38393, 116 +/- 26% . explant-1 . h-1;and 0.001 microM angiotensin II, 104 +/- 29% . explant-1 . h-1 ), the combination of dopamine and angiotensin II provoked a significant rise in AVP [420 +/- 83% . explant-1 . h-1 (P < 0.01)]. Similar results were observed with SKF-38393 and angiotensin II, and the AVP response was blocked to basal levels by either D1 or AT1 antagonism. These findings support a role for D1 receptor activation to increase AVP release and mediate angiotensin II-induced AVP release within the hypothalamo-neurohypophysial system. The data also suggest that the combined subthreshold stimulation of receptors that use distinct intracellular pathways can prompt substantial AVP release.

Excitatory amino acid receptors in the paraventricular hypothalamic nucleus mediate pressor response induced by carotid body chemoreceptor stimulation in rats

In urethane-anesthetized rats with spinal transection, antagonists of excitatory amino acid receptors, P2 purinoceptors and adrenoceptors were microinjected into the paraventricular hypothalamic nucleus (PVN) and their effects on the pressor response evoked by carotid body chemoreceptor stimulation were examined. Microinjections of the non-selective excitatory amino acid antagonist kynurenate, the non-NMDA receptor antagonist CNQX and the NMDA antagonist 2-amino-5-phosphonovalerate (AP5) into the PVN inhibited the chemoreceptor reflex-induced pressor response. The excitatory amino acid agonist L-glutamate injected into the PVN produced an increase in blood pressure. The P2 purinoceptor antagonist suramin did not affect the pressor response and ATP did not affect basal blood pressure. The alpha adrenoceptor antagonist phentolamine, prazosin and yohimbine also inhibited the chemoreceptor-induced pressor response, while the beta antagonist propranolol did not affect it. These findings indicate that excitatory amino acid receptors and alpha adrenoceptors in the PVN are involved in mediating the pressor response induced by carotid body chemoreceptor stimulation in rats.

The glutamatergic innervation of oxytocin- and vasopressin-secreting neurons in the rat supraoptic nucleus and its contribution to lactation-induced synaptic plasticity

The present ultrastructural study analysed the distribution of glutamatergic synapses on oxytocin- and vasopressin-secreting neurons in the rat supraoptic nucleus (SON) after post-embedding immunogold labelling for glutamate immunoreactivity, visible over synaptic-like vesicles, mitochondria and synaptic densities. Double labelling for glutamate and GABA showed that putative glutamatergic terminals were distinct from GABAergic terminals. In ultrathin sections stained for glutamate and either oxytocin or vasopressin, the proportion of glutamatergic synapses was similar on oxytocinergic and vasopressinergic somata in virgin rats under basal conditions of peptide release as well as in lactating rats, in which oxytocin secretion is enhanced. Cross-sectional soma areas were significantly increased in lactating rats: oxytocinergic profiles were, on average, approximately 40% larger than in virgin rats. However, the incidence of axo-somatic glutamatergic synapses (assessed as mean number of synapses per 100 microm of plasmalemma or proportion of somatic surface apposed to synaptic active zones) did not diminish, indicating that there was a compensatory increase of synapses during lactation. Also, we found an increase in the number of glutamatergic terminals making synaptic contact simultaneously onto two or more oxytocinergic elements in the same plane of section. Our observations therefore indicate that SON oxytocinergic and vasopressinergic neurons are innervated to a similar extent by a relatively large proportion of glutamatergic synapses. They reveal, moreover, that glutamatergic afferents participate in the lactation-induced synaptic plasticity of the oxytocinergic system.

Catecholaminergic innervation of the suprachiasmatic nucleus in the adult rat: ultrastructural relationships with neurons containing vasoactive intestinal peptide or vasopressin

Catecholaminergic fibers in the suprachiasmatic nucleus of adult rats were investigated by use of light- and electron-microscopic immunocytochemistry. The suprachiasmatic nucleus receives a modest density of tyrosine hydroxylase-containing axons, homogeneously distributed in the nucleus and forming varicosities throughout its entire rostro-caudal extension. Immunolabeling with antibodies against dopamine showed that this catecholamine input comprises a dopaminergic component. Many tyrosine hydroxylase-positive cells were localized at the immediate periphery of the suprachiasmatic nucleus. With electron-microscopic examination, dendrites of these neurons were found within the limits of the nucleus as well as at a border zone between the suprachiasmatic nucleus proper and the optic tract where they received unlabeled synapses, providing a morphological support for a possible role of dopaminergic neurons in the integration and/or transfer of light-related signals. More than 91% of catecholaminergic axonal varicosities were found to establish morphologically defined synapses with dendrites. To investigate whether these synapses might be shared with neurons of one or both of the two main peptidergic populations of the nucleus, namely vasoactive intestinal peptide- and vasopressin-containing neurons, we carried out double-labelling experiments combining immunoperoxidase and immunogold-silver labeling. Results showed only a few cases of direct association of the catecholaminergic terminals with these peptidergic categories. In both types of dually stained sections, catecholaminergic synapses were preferentially made with unlabeled dendrites. The homogeneous distribution of tyrosine hydroxylase-immunoreactive fibers in the suprachiasmatic nucleus could therefore reflect a lack of significant catecholaminergic innervation of both vasoactive intestinal peptide- and vasopressin-synthesizing neurons.

Taurine immunoreactivity in the rat supraoptic nucleus: prominent localization in glial cells

Taurine is an inhibitory amino acid that hyperpolarizes magnocellular neurosecretory neurons. To determine which cell types in the rat supraoptic nucleus contain taurine, we used a monoclonal antibody raised against a taurine conjugate. Preembedding immunocytochemistry was carried out at the light and electron microscopic levels using diaminobenzidine and gold-substituted silver-intensified peroxidase as markers. We report the presence of taurine in all cellular compartments of the supraoptic nucleus, except axons, with variable labeling intensities among the different compartments. Few cell bodies of magnocellular neurons were immunoreactive, but many distal dendrites and some proximal ones showed weak-to-moderate levels of immunoreactivity. Strong immunoreactivity was found over glial cell bodies and their processes, in particular in the ventral glial lamina of the supraoptic nucleus. Large astrocytic processes labeled with the taurine antibody included the endfeet participating in the glial limitans around capillaries and at the ventral surface of the hypothalamus. Other types of immunoreactive astrocytic profiles were found scattered within the neuropil where these processes participated in different interactions with the neuronal elements of the supraoptic nucleus. Immunoreactive glial expansions, sometimes even the main process of the glial cell, engulfed axonal boutons. Other labeled glial processes were found between two magnocellular perikarya or closely apposed to the membrane of axonal boutons contacting the neuronal cell bodies. The frequent finding of closely apposed glial and dendritic elements bearing different levels of taurine-like immunoreactivity suggests that exchange of taurine between those two compartments may occur. We propose that taurine could be released from supraoptic glia by a small decrease in osmolarity or by receptor-mediated mechanisms during conditions of low hormonal (vasopressin and/or oxytocin) needs. Such released taurine could then act on presynaptic or postsynaptic sites, or both, to exert its neuromodulatory actions.

Vasopressin and oxytocin gene expression in intact rats and under catecholamine deficiency during ontogenesis

The development of the hypothalamic vasopressin (VP) and oxytocin (OT) systems has been studied in rats from the 16th embryonic day (E16) until the 11th postnatal day (P11). The VP and OT mRNA-producing neurons were identified on cryostat sections by in situ hybridization using oligonucleotide probes labeled by [35S], [3H] or digoxigenin. Moreover, VP and OT gene expressions were evaluated either at E21 or at P11 following chronic depletion of catecholamines (CA). For this purpose, pregnant rats were daily injected with alpha-methyl-m(p)-tyrosine from gestational day 13 to 20 while neonates were daily injected with alpha-methyl-m(p)-tyrosine and neurotoxin 6-hydroxydopamine from postnatal day 2 to 10. No VP mRNA- or OT mRNA-expressing cells were observed in the hypothalamus of intact fetuses at E16, while 2 days later rather numerous VP and OT neurons occupied the anterior hypothalamus. One major bilateral group of VP and OT neurons was located in the supraoptic nucleus (SON). Less numerous labeled cells were found in the developing paraventricular nucleus (PVN). Some VP and OT neurons were also spread along the ventrolateral surface of the hypothalamus from the level of the median eminence, caudally, to the level of the optic nerves, rostrally. From E18 until birth, the OT neurons were localized in the dorsal portion of the SON, while its ventral portion was occupied by the VP neurons. The VP mRNA- and OT mRNA-expressing cells seemed to increase both in size and in number over the perinatal period. Frequent relatively long neuronal processes contained VP and OT mRNAs in fetuses and in newborns. When performed during the second half of the fetal life, the chronic depletion of CA did not cause any change in the VP and OT mRNA concentrations in the SON and PVN of fetuses. By contrast, similar treatment of neonates resulted in a significant increase of both mRNA levels in the SON. These data suggest that at least in the SON VP and OT gene expression might be under the inhibitory control of CA during the neonatal period.

Localization of tyrosine hydroxylase mRNA in the axons of the hypothalamo-neurohypophysial system

With in situ hybridization we examined the localization of mRNA coding for tyrosine hydroxylase (TH) in the rat hypothalamo-neurohypophysial system (HNS) under conditions of acute osmotic stress. Fifteen min after salt loading, hybridization signal of TH mRNA could be located in the magnocellular hypothalamic nuclei and in the median eminence (ME). In untreated animals, TH mRNA was detected only in the ME. In osmotically challenged animals that had been pretreated with colchicine, signals for TH mRNA remained confined to the ME, while pretreatment of salt loaded rats with a polymerase II transcription inhibitor resulted in labelling of the magnocellular perikarya but a decrease of the hybridization signal in the ME. Our results suggest that also TH mRNA is among the RNAs which are axonally transported in the HNS. TH mRNA can probably be stored in axons of the hypothalamo-neurohypophysial tract, to be transported retrogradely and translated upon certain stimuli.

Noradrenergic innervation of vasopressin- and oxytocin-containing neurons in the hypothalamic paraventricular nucleus of the macaque monkey: quantitative analysis using double-label immunohistochemistry and confocal laser microscopy

Previous reports on the rat and monkey hypothalamus have revealed a dense noradrenergic innervation within the hypothalamic paraventricular nucleus as assessed by dopamine-beta-hydroxylase immunohistochemistry. These single-label analyses were unable to delineate the cellular structures which receive this catecholaminergic innervation. Double-label preparations in the rat hypothalamic paraventricular nucleus have demonstrated synaptic interactions between noradrenergic varicosities and magnocellular neurons. However, the density and distribution of varicosities contacting chemically identified magnocellular neurons have not been assessed at the light or electron microscopic level. In this report, single-label immunohistochemistry was used to assess the morphology and distribution of vasopressin- and oxytocin-immunoreactive neurons within the macaque hypothalamic paraventricular nucleus. In addition, double-label immunohistochemistry was combined with confocal laser scanning microscopy to quantify the number of dopamine-beta-hydroxylase-immunoreactive varicosities in apposition to magnocellular neurons expressing vasopressin or oxytocin immunoreactivity. The morphology of chemically identified neurons was also compared to magnocellular neurons in the monkey hypothalamic paraventricular nucleus which were filled with Lucifer Yellow in order to assess the somatodendritic labeling of the immunohistochemical preparation. Qualitative assessment of immunohistochemically identified magnocellular cells indicated that vasopressin- and oxytocin-containing neurons are observed throughout the rostrocaudal extent of the monkey hypothalamic paraventricular nucleus, demarcating this structure from the surrounding anterior hypothalamus. The distribution of the two nonapeptides is complementary, with vasopressin-immunoreactive neurons having a greater somal volume and located in a more medial aspect of the mid and caudal hypothalamic paraventricular nucleus relative to oxytocin-immunoreactive perikarya. For the double-label preparations, a series of confocal optical sections was assessed through the total somal volume of vasopressin- and oxytocin-immunoreactive neurons along with the corresponding dopamine-beta-hydroxylase-immunoreactive varicosities in the same volume of tissue, generating a varicosity-to-neuron ratio which was further characterized morphologically to assess afferent input to the soma and proximal dendrites. Quantitative analysis revealed that vasopressin-immunoreactive neurons received approximately two thirds of their dopamine-beta-hydroxylase-immunoreactive varicosities in apposition to the proximal dendrites and one third in apposition to the somata. Furthermore, vasopressin-immunoreactive neurons received a greater innervation density than oxytocin-immunoreactive neurons, which did not have a differential distribution of varicosities on the proximal dendrites and somata. The distribution of dopamine-beta-hydroxylase-immunoreactive afferents on magnocellular neurons in the hypothalamic paraventricular nucleus may reflect a physiological role of this circuit in terms of preferential release of vasopressin from magnocellular neurons upon noradrenergic stimulation.

Comparative single and double immunolabelling with antisera against catecholamine biosynthetic enzymes: criteria for the identification of dopaminergic, noradrenergic and adrenergic structures in selected rat brain areas

Immunodetection of catecholamine biosynthetic enzymes is frequently used for the visualization of central nervous catecholaminergic systems. Because of the method's limited specificity for the different catecholamines, interpretation of the results often presents difficulties. To determine criteria for the identification of dopaminergic, noradrenergic, and adrenergic afferents to the rat amygdaloid complex, comparative immunolabelling for tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyl-transferase (PNMT) was carried out using single- and double-labelling for fluorescence, light- and electron microscopy. The observations were complemented by findings in brainstem and hypothalamic areas. The results indicated that TH-labelling detected preferentially dopaminergic afferents in the lateral central and intercalated amygdaloid nuclei. DBH-labelling detected noradrenergic axons in nuclei lacking PNMT-immunoreactive fibres, and PNMT was a marker for adrenergic axons in the entire complex. For nuclei with combined dense dopaminergic, noradrenergic and/or adrenergic innervation, morphological and immunolabelling characteristics were described which, to a certain extent, enabled identification of the different afferents in anti-TH or anti-DBH-preparations. Using a monoclonal TH-antiserum, noradrenergic and adrenergic axons displayed weaker immunoreactivity than dopaminergic ones, and possessed characteristic morphological features. TH-immunoreactivity in noradrenergic axons differed depending on their origin, and showed intra-axonal compartmentalization. The present study provides a basis for the use of the detection of biosynthetic enzymes in future investigations into the ultrastructure and connectivity of the catecholaminergic amygdala innervation.

Cocaine-induced elevation of plasma corticosterone is mediated by different neurotransmitter systems in rats

It has previously been demonstrated that cocaine stimulates the hypothalamic-pituitary-adrenal (HPA) axis through hypothalamic corticotropin-releasing factor (CRF) secretion. The role of different neurotransmitters in mediation of the cocaine-induced elevation of plasma corticosterone (CORT) were investigated in rats by using transmitter antagonists. Peripheral (IP) pretreatment with a dopaminergic antagonist, pimozide (0.01-1.0 mg/kg, IP), a noradrenergic blocker, phenoxybenzamine (1.0-4.0 mg/kg, IP), a beta-adrenergic blocker, propranolol (0.2-10 mg/kg, IP), an opiate antagonist, naloxone (1.0-4.0 mg/kg, IP), and a muscarinic cholinergic antagonist, atropine (1.0-4.0 mg/kg, IP), inhibited the cocaine-induced CORT response dose dependently. A similar dose-dependent inhibition of the plasma CORT response induced by cocaine was observed after the ICV route of administration of these antagonists in microgram quantities. None of the investigated IP or ICV doses of transmitter antagonists altered the basal CORT level. These results suggest that the activation of multiple neurotransmitter systems, including catecholaminergic, opiate, and cholinergic systems, might be responsible for the cocaine-induced HPA axis activation, probably through the specific receptors located in the CNS.

Evaluation for roles of brain prostaglandins in the catecholamine-induced vasopressin secretion in conscious rats

To evaluate roles of prostaglandins (PGs) in vasopressin (AVP) secretion elicited by stimulating alpha-adrenergic and dopaminergic receptors in the periventricular region, we examined in conscious rats the effects of intracerebroventricular (i.c.v.) injections of a cyclooxygenase inhibitor meclofenamate on the plasma AVP responses to i.c.v. applications of angiotensin II (ANG II), phenylephrine and dopamine. I.c.v. injections of 58 pmol ANG II produced, 5 and 15 min later, augmentations of plasma AVP accompanied by elevations of arterial pressure and tendencies of reduction in heart rate. Similarly, the administrations of 0.53 mumol phenylephrine or dopamine enhanced plasma AVP 5 min later, without altering arterial pressure and heart rate significantly. Meclofenamate (0.31 mumol) applied i.c.v. 30 min prior to the administrations of ANG II remarkably inhibited the AVP and pressor responses to this peptide. However, the responses of plasma AVP, arterial pressure and heart rate to phenylephrine or dopamine were not affected by the i.c.v. administrations of 0.31 mumol meclofenamate. The injections of meclofenamate followed by the administrations of a vehicle for ANG II and the catecholamines were without effect on plasma AVP and the cardiovascular parameters. Plasma osmolality, sodium, potassium and chloride in all the groups mentioned above were not significantly changed during experiments. These results suggest that PGs generated in the periventricular region, despite their probable stimulatory roles in the ANG II-evoked AVP secretion, may not participate in the AVP-releasing mechanisms activated by dopaminergic and alpha-adrenergic receptors, supporting the view that PGs and the catecholamines may facilitate AVP release via separate pathways.

Coexistence of oxytocin and tyrosine hydroxylase in the rat hypothalamus, an immunocytochemical study

Immunocytochemical double labelling was used to determine the structural relationship of oxytocin (OT) and tyrosine hydroxylase (TH) containing perikarya and processes in the rat hypothalamus. Extrahypothalamic TH fibers, as well as parvocellular TH neurons were found to form contacts with OT cells. A fraction of the OT neurons contained TH immunoreactivity. It is likely that in addition to the classical mesencephalic afferences also hypothalamic interneurons and magnocellular dopaminergic neurons control the hypothalamo neurohypophysial system.

Noradrenergic innervation of the hypothalamus of rhesus monkeys: distribution of dopamine-beta-hydroxylase immunoreactive fibers and quantitative analysis of varicosities in the paraventricular nucleus

The distribution of noradrenergic processes within the hypothalamus of rhesus monkeys (Macaca mulatta) was examined by immunohistochemistry with an antibody against dopamine-beta-hydroxylase. The results revealed that the pattern of dopamine-beta-hydroxylase immunoreactivity varied systematically throughout the rhesus monkey hypothalamus. Extremely high densities of dopamine-beta-hydroxylase-immunoreactive processes were observed in the paraventricular and supraoptic nuclei, while relatively lower levels were found in the arcuate and dorsomedial nuclei and in the medial preoptic, perifornical, and suprachiasmatic areas. Moderate levels of dopamine-beta-hydroxylase immunoreactivity were found throughout the lateral hypothalamic area and in the internal lamina of the median eminence. Very few immunoreactive processes were found in the ventromedial nucleus or in the mammillary complex. Other midline diencephalic structures were found to have high densities of dopamine-beta-hydroxylase immunoreactivity, including the paraventricular nucleus of the thalamus and a discrete subregion of nucleus reuniens, the magnocellular subfascicular nucleus. A moderate density of dopamine-beta-hydroxylase immunoreactive processes were found in the rhomboid nucleus and zona incerta whereas little dopamine-beta-hydroxylase immunoreactivity was found in the fields of Forel, nucleus reuniens, or subthalamic nucleus. The differential distribution of dopamine-beta-hydroxylase-immunoreactive processes may reflect a potential role of norepinephrine as a regulator of a variety of functions associated with the nuclei that are most heavily innervated, e.g., neuroendocrine release from the paraventricular and supraoptic nuclei, and gonadotropin release from the medial preoptic area and mediobasal hypothalamus. Additionally, quantitative analysis of dopamine-beta-hydroxylase-immunoreactive varicosities was performed on a laser scanning microscope in both magnocellular and parvicellular regions of the paraventricular nucleus of the hypothalamus. The methodology employed in this study allowed for the high resolution of immunoreactive profiles through the volume of tissue being analyzed, and was more accurate than conventional light microscopy in terms of varicosity quantification. Quantitatively, a significant difference in the density of dopamine-beta-hydroxylase-immunoreactive varicosities was found between magnocellular and parvicellular regions, suggesting that parvicellular neurons received a denser noradrenergic input. These differential patterns may reflect an important functional role for norepinephrine in the regulation of anterior pituitary secretion through the hypothalamic-pituitary-adrenal stress axis.

Immunohistochemical study of the catecholaminergic innervation of the spinal cord of the rat using specific antibodies against dopamine and noradrenaline

We have assessed the relative contributions of dopaminergic and noradrenergic descending systems to the catecholaminergic innervation of the rat spinal cord. Fibres and terminals were labelled with their own neurotransmitter by using specific antibodies raised against dopamine (DA) and noradrenaline (NA) respectively. For this purpose, immunohistochemistry according to the peroxidase anti-peroxidase technique was performed in different experimental conditions. Two group of rats received intracisternal 6-hydroxy-dopamine (6-OHDA) injections either with or without benzatropine pretreatment. Animals of a third group were not pretreated at all. While 6-OHDA induced a complete disappearance of spinal NA-like immunoreactivity (NA-LI), except for scarce residual fibres in the thoracic intermedio-lateral cell column, DA-like immunoreactivity (DA-LI) was unaffected by the lesion. This strongly suggests that the antisera used specifically labelled NA-containing and DA-containing fibres respectively. Spinal DA-LI and NA-LI innervations differed markedly in their topographical distributions and in the morphology of the corresponding fibres. DA-LI innervation was restricted to laminae I, III and IV and to the intermediate zone, especially the autonomic areas. In the ventral horn, it was sparse and more visible after acidification of the fixation solution. NA-LI innervation was much more widely spread. In addition, the organization of NA-LI fibres suggests that the innervation of the whole dorsal horn comes from a group of fibres travelling, at least partially, in the superficial dorsal horn.

Noradrenergic innervation of vasopressin-containing neurons in the rat hypothalamic supraoptic nucleus

The noradrenergic innervation of vasopressin (VP)-containing neurons in the supraoptic nucleus (SON) of the rat hypothalamus was studied electron microscopically by using double-labeling immunocytochemistry combining the pre-embedding peroxidase-anti-peroxidase method with post-embedding immunocolloidal gold staining. Noradrenaline-like immunoreactive axon terminals were found to make synaptic contacts with neurophysin II-like immunoreactive neurons in the SON. This provides morphological evidence for noradrenergic control of neuronal activity of VP-containing neurons at the SON level.

Dopamine D2 receptor activation depolarizes rat supraoptic neurones in hypothalamic explants

1. Intracellular current and voltage clamp recordings were obtained from rat supraoptic nucleus neurones in superfused hypothalamic explants in order to evaluate their response to dopamine and to D1 and D2 agonists. 2. With one exception, exposure to dopamine (10-200 microM) depolarized supraoptic neurones. When tested for an effect on twenty-one spontaneously active supraoptic neurones, dopamine enhanced the firing of all eleven continuous-firing (possibly oxytocin-secreting) neurones and prolonged the burst in all ten phasic-firing (vasopressin-secreting) neurones. 3. In sixty-seven of sixty-eight neurones where current injection was used to maintain membrane potential below threshold for action potential generation, current clamp data revealed that exposure to dopamine (10-200 microM) was followed in 10-17 s by a gradual 3-7 mV membrane depolarization that lasted for 4-15 min and was accompanied by a 12-23% reduction in input resistance. Exposure to quinpirole, a D2 agonist (10-200 microM), induced a similar response with comparable onset, duration and change in input resistance. In contrast, tests on sixteen cells indicated little or no response to a D1 agonist SKF38393. 4. Under voltage clamp, dopamine was noted to induce an inward current, accompanied by a 7.5-40% increase in membrane conductance over the corresponding time course. 5. Voltage-current plots for dopamine-induced depolarizations were linear in the range -50 to -110 mV. Dopamine and quinpirole depolarizations had extrapolated mean reversal potentials of -25 +/- 10 mV (mean +/- S.D.) and -20 +/- 15 mV respectively. This approximated the mean reversal potential of -20 +/- 8 mV measured from the dopamine-induced inward current using single-electrode voltage clamp. 6. The actions of dopamine were selectively antagonized by two D2 receptor antagonists, sulpiride and spiperone, but neither influenced membrane depolarizations induced by equimolar concentrations of noradrenaline. Dopamine-induced depolarizations also persisted following selective blockade of alpha 1-adrenergic receptors by prazosin; under these conditions, noradrenaline induced membrane hyperpolarization. 7. Following complete substitution of external Na+ with Tris, the reversal potential for the dopamine-induced response was shifted to -70 +/- 9.8 mV. This value was consistently less negative than the estimated potassium equilibrium potential. 8. The depolarization action of dopamine persisted in media containing tetrodotoxin and with an external calcium concentration ([Ca2+]o) of 0 mM-Ca2+ with 6 mM-Mg2+ or Mn2+, but was abolished following intracellular injection of [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a Ca2+ chelator.(ABSTRACT TRUNCATED AT 400 WORDS)

Noradrenaline-like terminals in the cat nucleus ventralis posterior of the thalamus

Noradrenaline-like immunoreactivity in the cat nucleus ventralis posterior of the thalamus was investigated using an indirect immunocytochemical technique. Specific antinoradrenaline antibodies, raised in rabbits, were used. It was first verified that these antibodies recognize noradrenaline cells bodies of the locus coeruleus and their ascending axons in the ascending noradrenergic tract. In the nucleus ventralis posterior itself, noradrenaline-like fibers were observed. They were either randomly distributed or grouped around nonlabeled cell bodies. These neurons were generally oblong and measured 60-80 microns. With electron microscopy, preliminary results showed immunoreactive fibers in close apposition to unlabeled cell bodies or dendrites. The precise nature of these profiles was sometimes difficult to ascertain, since experiments were done in presence of detergent. In some cases symmetric synapses might be observed between immunoreactive axon terminals and unlabeled dendrites. The specificity of the reaction is discussed in the light of several control experiments.

GABA: a dominant neurotransmitter in the hypothalamus

To study the organization and distribution of the inhibitory amino acid neurotransmitter GABA in the medial hypothalamus, we used a postembedding immunocytochemical approach with colloidal gold. Quantitative analysis showed that half (49%) of all synapsing boutons studied were immunoreactive for GABA, based on immunogold staining of the suprachiasmatic, arcuate, supraoptic, and paraventricular nuclei. This was corroborated with pre-embedding peroxidase immunostaining with antisera against glutamate decarboxylase, the GABA synthetic enzyme. These data suggest that GABA is the numerically dominant neurotransmitter in the hypothalamus, and emphasize the importance of inhibitory circuits in the hypothalamus. Serial ultrathin sections were used to reconstruct GABA immunoreactive boutons and axons in three dimensions. With this type of analysis we found less morphological heterogeneity between GABA immunoreactive boutons than with single ultrathin sections. Single sections sometimes showed boutons containing only small clear vesicles, and other with both clear vesicles and small dense core vesicles. However, with serial sections through individual boutons, dense core vesicles were consistently found at the periphery of the pre-synaptic GABA immunoreactive boutons, suggesting probable co-localization of GABA with unidentified peptides in most if not all boutons throughout the hypothalamus. A positive correlation was found between the density of small clear vesicles and the intensity of immunostaining with colloidal gold particles. GABA immunoreactive axons generally made symmetrical type synaptic specializations, although a small percentage made strongly asymmetrical synaptic specializations. Vesicles in GABA immunoreactive boutons were slightly smaller than those in non-reactive boutons. Synaptic efficacy is related to the position of the synapse on the post-synaptic neuron. While the majority of GABA immunoreactive axons made synaptic contact with dendrites, the distribution of GABA immunoreactive synapses on somata and dendrites was the same as would be expected from a random distribution of all boutons. No preferential innervation of cell bodies by GABA immunoreactive terminals was found. Serial ultrathin sections showed that a GABA immunoreactive axon would sometimes make repeated synaptic contacts with a single postsynaptic neuron, indicating a high degree of direct control by the presynaptic GABAergic cell. Other immunoreactive axons made synaptic contact with a number of adjacent dendrites and cells, suggesting a role for GABA in synchronizing the activity of hypothalamic neurons. Based on the density of immunogold particles per unit area, varying concentrations of immunoreactive GABA were found in different presynaptic boutons in the hypothalamus.

Ultrastructural demonstration of dopamine-beta-hydroxylase immunoreactive nerve terminals on vasopressin neurons in the paraventricular nucleus of the rat by double-labeling immunocytochemistry

The noradrenergic innervation of vasopressin (VP) neurons in the paraventricular nucleus (PVN) of the rat was studied ultrastructurally by double-labeling immunocytochemistry combining the preembedding peroxidase-antiperoxidase method for dopamine-beta-hydroxylase (DBH) with the post-embedding immunogold staining method for neurophysin II, the carrier protein of VP. DBH-like immunoreactive nerve terminals were found to make synaptic contacts with neurophysin II-like immunoreactive neuronal perikarya and their processes. This provides morphological evidence for noradrenergic control of the release of VP, at the PVN of the rat.

Monoamine synaptic structure and localization in the central nervous system

The monoamines dopamine, noradrenaline, adrenaline, and serotonin as well as the diamine histamine have a widespread distribution in the central nervous system within synaptic terminals and nonsynaptic varicosities. In certain regions of the central nervous system the monoamines are contained in varicosities that have no synaptic specialization associated with them, suggesting a possible neuromodulatory role for some of the monoamines. The majority of monoamine labelled structures are synaptic terminals which are characterized by the presence of small, clear vesicles (40-60 nm) and large, granular vesicles (70-120 nm) within the terminal. A third population of vesicles--small, granular vesicles--which are visible only after histochemical staining, are probably the equivalent of the small, clear vesicles present after either autoradiographic or immunohistochemical labelling. Most monoamine containing terminals contact dendrites and dendritic spines and, less frequently, neuronal somata and other axons. Both asymmetrical and symmetrical membrane specializations are associated with monoaminergic terminals; however, asymmetrical contacts are the most frequent type found. These ultrastructural results indicate that monoamine containing terminals and varicosities in general share many common morphological features, but still have diverse functions.

Inhibitory role of periventricular dopaminergic mechanisms in hemorrhage-induced vasopressin secretion in conscious rats

Acute blood loss (16 ml/kg b. wt.) in conscious rats caused, 5 min later, increases in plasma vasopressin (AVP) concentration accompanied by reductions in arterial pressure and hematocrit. The plasma AVP response was markedly enhanced by intracerebroventricular injection (10 microliters) of a dopamine antagonist, haloperidol (0.15 mumol), which did not affect the responses of arterial pressure and hematocrit significantly. These results suggest that periventricular dopaminergic mechanisms may act to inhibit hemorrhage-induced AVP secretion.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

Association of dopaminergic fibers with corticotropin releasing hormone (CRH)-synthesizing neurons in the paraventricular nucleus of the rat hypothalamus

Catecholamines are known to exert a central influence on the hypothalamo-hypophyseal-adrenal neuroendocrine system. The selective dopaminergic innervation of the hypothalamic paraventricular nucleus (PVN) and putative relationships between dopaminergic fibers and corticotropin releasing hormone (CRH)-synthesizing neurons were studied in the male rat by means of immunocytochemistry following the elimination of noradrenergic and adrenergic inputs to the hypothalamus. A 3.0-mm-wide coronal cut was placed unilaterally in the brain at the rostral level of the mesencephalon. All neuronal structures from the cortex to the ventral surface of the brainstem, including the ascending catecholaminergic fiber bundles were transected. This surgical intervention resulted in the accumulation of dopamine-beta-hydroxylase (DBH)-immunoreactivity in axons proximal to the cut, and an almost complete disappearance of DBH activity in those located distal to the lesion. Two weeks following the operation, DBH immunoreactivity was significantly diminished in the PVN located on the side of lesion, while tyrosine hydroxylase (TH)-immunoreactivity was present in a substantial number of fibers in the same nucleus. Both DBH- and TH-immunoreactive axons were preserved in the contralateral PVN. Simultaneous immunocytochemical localization of either DBH- or TH-IR fibers and corticotropin releasing hormone-synthesizing neurons in the hypothalami from brainstem-lesioned, colchicine treated animals revealed that the distribution of catecholaminergic fibers and CRH neurons is homologous within the PVN of the intact side. Only a few scattered DBH-immunoreactive axons were detected among CRH-producing neurons in the PVN on the side of the lesion. In contrast, many tyrosine hydroxylase containing neurons and neuronal processes were observed on the lesioned side and the TH-IR fibers established juxtapositions with CRH-synthesizing neurons.(ABSTRACT TRUNCATED AT 250 WORDS)