Characterization of hypothalamic noradrenaline receptors in the supraoptic nucleus and periventricular region of the paraventricular nucleus of mice in vitro

Activation of alpha-1 adrenergic receptors increases cytosolic calcium in neurones of the paraventricular nucleus of the hypothalamus

Norepinephrine (NE) activates adrenergic receptors (ARs) in the hypothalamic paraventricular nucleus (PVN) to increase excitatory currents, depolarise neurones and, ultimately, augment neuro-sympathetic and endocrine output. Such cellular events are known to potentiate intracellular calcium ([Ca²⁺ ]_i ); however, the role of NE with respect to modulating [Ca²⁺ ]_i in PVN neurones and the mechanisms by which this may occur remain unclear. We evaluated the effects of NE on [Ca²⁺ ]_i of acutely isolated PVN neurones using Fura-2 imaging. NE induced a slow increase in [Ca²⁺ ]_i compared to artificial cerebrospinal fluid vehicle. NE-induced Ca²⁺ elevations were mimicked by the α₁ -AR agonist phenylephrine (PE) but not by α₂ -AR agonist clonidine (CLON). NE and PE but not CLON also increased the overall number of neurones that increase [Ca²⁺ ]_i (ie, responders). Elimination of extracellular Ca²⁺ or intracellular endoplasmic reticulum Ca²⁺ stores abolished the increase in [Ca²⁺ ]_i and reduced responders. Blockade of voltage-dependent Ca²⁺ channels abolished the α₁ -AR induced increase in [Ca²⁺ ]_i and number of responders, as did inhibition of phospholipase C inhibitor, protein kinase C and inositol triphosphate receptors. Spontaneous phasic Ca²⁺ events, however, were not altered by NE, PE or CLON. Repeated K⁺ -induced membrane depolarisation produced repetitive [Ca²⁺ ]_i elevations. NE and PE increased baseline Ca²⁺ , whereas NE decreased the peak amplitude. CLON also decreased peak amplitude but did not affect baseline [Ca²⁺ ]_i . Taken together, these data suggest receptor-specific influence of α₁ and α₂ receptors on the various modes of calcium entry in PVN neurones. They further suggest Ca²⁺ increase via α₁ -ARs is co-dependent on extracellular Ca²⁺ influx and intracellular Ca²⁺ release, possibly via a phospholipase C inhibitor-mediated signalling cascade.

Alpha1- and alpha2-containing GABAA receptor modulation is not necessary for benzodiazepine-induced hyperphagia

Benzodiazepines increase food intake, an effect attributed to their ability to enhance palatability. We investigated which GABA(A) receptor subtypes may be involved in mediating benzodiazepine-induced hyperphagia. The role of the alpha2 subtype was investigated by observing the effects of midazolam, on the behavioural satiety sequence in mice with targeted deletion of the alpha2 gene (alpha2 knockout). Midazolam (0.125, 0.25 and 0.5mg/kg) increased food intake and the amount of time spent feeding in alpha2 knockout mice, suggesting that BZ-induced hyperphagia does not involve alpha2-containing GABA(A) receptors. We further investigated the roles of alpha1- and alpha3-containing GABA(A) receptors in mediating BZ-induced hyperphagia. We treated alpha2(H101R) mice, in which alpha2-containing receptors are rendered benzodiazepine insensitive, with L-838417, a compound which acts as a partial agonist at alpha2-, alpha3- and alpha5-receptors but is inactive at alpha1-containing receptors. L-838417 (10 and 30 mg/kg) increased food intake and the time spent feeding in both wildtype and alpha2(H101R) mice, demonstrating that benzodiazepine-induced hyperphagia does not require alpha1- and alpha2-containing GABA(A) receptors. These observations, together with evidence against the involvement of alpha5-containing GABA(A) receptors, suggest that alpha3-containing receptors mediate BZ-induced hyperphagia in the mouse.

Mechanisms involved in the pressor response to noradrenaline microinjection into the supraoptic nucleus of unanesthetized rats

We report on the cardiovascular effects of noradrenaline (NA) microinjection into the hypothalamic supraoptic nucleus (SON) as well as the central and peripheral mechanisms involved in their mediation. Microinjections of NA 1, 3, 10, 30 or 45 nmol/100 nL into the SON caused dose-related pressor and bradycardiac response in unanesthetized rats. The response to NA 10 nmol was blocked by SON pretreatment with 15 nmol of the alpha(2)-adrenoceptor antagonist RX821002 and not affected by pretreatment with equimolar dose of the selective alpha(1)-adrenoceptor antagonist WB4101, suggesting that local alpha(2)-adrenoceptors mediate these responses. Pretreatment of the SON with the nonselective beta-adrenoceptor antagonist propranolol 15 nmol did not affect the pressor response to NA microinjection of into the SON. Moreover, the microinjection of the 100 nmol of the selective alpha(1)-adrenoceptor agonist methoxamine (MET) into the SON did not cause cardiovascular response while the microinjection of the selective alpha(2)-adrenoceptor agonists BHT920 (BHT, 100 nmol) or clonidine (CLO, 5 nmol) caused pressor and bradycardiac responses, similar to that observed after the microinjection of NA. The pressor response to NA was potentiated by intravenous pretreatment with the ganglion blocker pentolinium and was blocked by intravenous pretreatment with the V(1)-vasopressin receptor antagonist dTyr(CH2)5(Me)AVP, suggesting an involvement of circulating vasopressin in this response. In conclusion, our results suggest that pressor responses caused by microinjections of NA into the SON involve activation of local alpha(2)-adrenoceptor receptors and are mediated by vasopressin release into circulation.

Inhibition of dopamine and norepinephrine reuptake produces additive effects on energy balance in lean and obese mice

Although originally developed as an antidepressant, long-term bupropion (BUP) treatment was recently shown to cause 5-8% weight loss over placebo in clinical trials with obese adults. BUP's antidepressant properties probably stem from its ability to increase extracellular brain dopamine (DA) and norepinephrine (NE) levels by inhibiting their reuptake, although the mechanism of BUP-induced weight loss is unknown. Consequently, the acute effects of DA and NE reuptake inhibition on energy homeostasis were determined by measuring food intake and body weight in mice following peripheral (intraperitoneal (i.p.)) administration of either BUP, a selective DA (GBR12783), or a selective NE (nisoxetine (NIS)) reuptake inhibitor. BUP, GBR12783, and NIS all dose-dependently decreased acute food intake in fasted lean mice. The ability of BUP to decrease food intake was independent of its ability to cause a temporary increase in locomotor activity. The inhibitory effects of acute GBR12783 and NIS on short-term food intake were additive. Subchronic (via mini-osmotic pump) administration of GBR12783 and NIS produced a transient nonadditive effect on food intake, but produced an additive reduction in body weight (8-10%). Because obesity can affect catecholaminergic signaling, we determined the effects of i.p. BUP, GBR12783, and NIS on short-term food intake in obese mice. Acute BUP, GBR12783, and NIS dose-dependently reduced acute food intake, and the additive effect of GBR12783 and NIS on acute food intake was preserved in obese mice. These results demonstrate that combined DA and NE reuptake inhibition produces additive effects on energy balance in lean and obese mice on both standard and high-fat diet, providing a foundation for further research on the effects of BUP and similar compounds on energy balance in mice.

Neuropeptide Y, alpha-melanocyte-stimulating hormone, and monoamines in food intake regulation

Obesity is increasing in severity and prevalence in the United States and represents a major public health issue. No effective pharmacologic treatment leading to sustained weight loss currently exists. The growing interest in the regulation of food intake stems from the current drug treatments for obesity, almost all of which interfere with the monoamine system. Our knowledge of potential interactions between the orexigenic and anorexigenic pathways is limited and fragmented, making the development of targeted drug therapy for obesity difficult. The present review of the interaction of neuropeptides and monoamines emphasizes the complexity of the central mechanisms that regulate feeding behavior. Two main systems are implicated in food intake regulation: neuropeptide Y (NPY) and pro-opiomelanocortin. alpha-Melanocyte-stimulating hormone is a tridecapeptide cleaved from pro-opiomelanocortin that acts to inhibit food intake. The predominant NPY orexigenic receptors are NPY-Y1 and NPY-Y5, and the two anorexigenic melanocortin receptors involved in hypothalamic food intake control are MC3-R and MC4-R. Both neuropeptides interact with monoamines in the hypothalamus to control physiologic states such as hunger, satiation, and satiety. Serotonin suppresses food intake and body weight, acting mainly through the serotonin 1B receptor. Dopamine regulates hunger and satiety by acting in specific hypothalamic areas, through the D1 and D2 receptors. Noradrenaline activation of alpha1- and beta2-adrenoceptors decreases food intake, and stimulation of the alpha2-adrenoceptor increases food intake. A better understanding of the detailed mechanisms underlying the pathogenesis of hyperphagia and hypophagia is needed to develop new therapeutic approaches to obesity.

Central noradrenergic mechanisms underlying acute stress responses of the Hypothalamo-pituitary-adrenal axis: adaptations through pregnancy and lactation

Hypothalamo-pituitary-adrenal axis responses to stress are attenuated perinatally, and may contribute towards conservation of energy stores and/or prevention of overexposure to glucocorticoid and its adverse effects in the developing fetus/neonate. Previous work has shown that reduced central drive to the hypothalamo-pituitary-adrenal axis is responsible, since parvocellular paraventricular nucleus neurone responses are reduced. One of the main input pathways to the paraventricular nucleus that is activated by the majority of stressors is the brainstem noradrenergic system. This review outlines key noradrenergic mechanisms that mediate hypothalamo-pituitary-adrenal axis responses to acute stress, and addresses aspects of their adaptation in pregnancy and lactation that can explain the stress hyporesponsiveness at that time. In summary, reduced noradrenaline release and adrenergic receptor expression in the paraventricular nucleus may lead to reduced sensitivity of the hypothalamo-pituitary-adrenal axis to adrenergic antagonists and agonists and its responses to stress. While there are subtle differences in these changes between pregnancy and lactation, it would appear that reduced effectiveness of the noradrenergic input can at least partly account for the reduced hypothalamo-pituitary-adrenal axis responses both pre- and post-natally.

Reduced activity of the noradrenergic system in the paraventricular nucleus at the end of pregnancy: implications for stress hyporesponsiveness

We investigated whether changes in noradrenaline neurotransmission in the hypothalamus could explain the hyporesponsiveness of the hypothalamic-pituitary-adrenal (HPA) axis in late pregnancy. Noradrenaline release within the hypothalamic paraventricular nucleus in response to swim stress, as estimated by microdialysis and high-performance liquid chromatography, was lower in 20-day pregnant rats compared to virgin rats. Driving a central noradrenergic pathway using intravenous cholecystokinin increased adrenocorticotropic hormone (ACTH) secretion in virgin rats, but the response was significantly less in 16-day and 20-day pregnant rats. Thus, the activity of noradrenergic inputs to the paraventricular nucleus and the HPA axis is attenuated in late pregnancy. The sensitivity of the HPA axis to noradrenaline in pregnancy was investigated by intracerebroventricular administration of an alpha1-receptor antagonist, benoxathian, before and during exposure to swim stress. In virgin rats, benoxathian increased basal and stress-induced ACTH secretion, but in late pregnant rats the benoxathian effects were attenuated, indicating reduced sensitivity of the HPA axis to noradrenaline neurotransmission and/or the inability of the system to become disinhibited at this time. alpha1A-adrenoreceptor mRNA expression in the parvocellular and magnocellular paraventricular nucleus, measured by in situ hybridisation, was decreased in late pregnant compared to virgin rats. Additionally, blocking endogenous opioid inhibition with naloxone pretreatment restored the ACTH secretory response to cholecystokinin in pregnant rats. Thus, in late pregnancy, there is reduced noradrenergic input to the paraventricular nucleus and reduced alpha1A-receptor expression in the paraventricular nucleus, both of which may contribute to the reduced responsiveness of the HPA axis in pregnancy.

Association between a vasopressin receptor AVPR1A promoter region microsatellite and eating behavior measured by a self-report questionnaire (Eating Attitudes Test) in a family-based study of a nonclinical population

OBJECTIVES

Considerable evidence including twin and family studies suggests that biologic determinants interact with cultural cues in the etiology of anorexia and bulimia nervosa. A gene that makes "biologic sense" in contributing susceptibility to these disorders, and to our knowledge not previously investigated for this phenotype, is the vasopressin receptor (AVPR1A), which we have tested for association with eating pathology.

METHODS

We genotyped 280 families with same-sex siblings for two microsatellites in the promoter region of the AVPR1A gene. Siblings completed the 26-item Eating Attitudes Test (EAT) and the Drive for Thinness (DT) and Body Dissatisfaction (BD) subscales of the Eating Disorders Inventory (EDI). The Quantitative Transmission Disequilibrium Test program (QTDT), which employs flexible and powerful variance-components procedures, was used to test for an association between EAT scores and the two AVPR1A promoter region microsatellites, RS1 and RS3.

RESULTS

A significant association (p = .036) was detected between the RS3 microsatellite and EAT scores. The strongest association was between RS3 and the Dieting subscale of the EAT (p = .011). A significant association was also observed between the EDI-DT and the RS3 microsatellit (p = .0450).

CONCLUSIONS

We demonstrate for the first time an association between a microsatellite polymorphism in the AVPR1A promoter region and scores on the EAT as well as with the EDI-DT. The strongest association was observed between the RS3 microsatellite and the Dieting subscale of the EAT. The relevant phenotype appears to tap severe dietary restriction for weight loss purposes.

Cardiovascular and renal sympathetic activation by blood-borne TNF-alpha in rat: the role of central prostaglandins

In pathophysiological conditions, increased blood-borne TNF-alpha induces a broad range of biological effects, including activation of the hypothalamic-pituitary-adrenal axis and sympathetic drive. In urethane-anesthetized adult Sprague-Dawley rats, we examined the mechanisms by which blood-borne TNF-alpha activates neurons in paraventricular nucleus (PVN) of hypothalamus and rostral ventrolateral medulla (RVLM), two critical brain regions regulating sympathetic drive in normal and pathophysiological conditions. TNF-alpha (0.5 microg/kg), administered intravenously or into ipsilateral carotid artery (ICA), activated PVN and RLVM neurons and increased sympathetic nerve activity, arterial pressure, and heart rate. Responses to intravenous TNF-alpha were not affected by vagotomy but were reduced by mid-collicular decerebration. Responses to ICA TNF-alpha were substantially reduced by injection of the cyclooxygenase inhibitor ketorolac (150 microg) into lateral ventricle. Injection of PGE(2) (50 ng) into lateral ventricle or directly into PVN increased PVN or RVLM activity, respectively, and sympathetic drive, with shorter onset latency than blood-borne TNF-alpha. These findings suggest that blood-borne cytokines stimulate cardiovascular and renal sympathetic responses via a prostaglandin-dependent mechanism operating at the hypothalamic level.

Noradrenaline excites and inhibits GABAergic transmission in parvocellular neurons of rat hypothalamic paraventricular nucleus

Noradrenaline (NA) is a major neurotransmitter that regulates many neuroendocrine and sympathetic autonomic functions of the hypothalamic paraventricular nucleus (PVN). Previously NA has been shown to increase the frequency of excitatory synaptic activity of parvocellular neurons within the PVN, but little is known about its effects on inhibitory synaptic activity. In this work, we studied the effects of NA (1-100 microM) on the spontaneous inhibitory synaptic currents (sIPSC) of type II PVN neurons in brain slices of the rat using the whole cell patch-clamp technique. Spontaneous IPSCs were observed from most type II neurons (n = 121) identified by their anatomical location within the PVN and their electrophysiological properties. Bath application of NA (100 microM) increased sIPSC frequency by 256% in 59% of the neurons. This effect was blocked by prazosin (2-20 microM), the alpha(1)-adrenoceptor antagonist and mimicked by phenylephrine (10-100 microM), the alpha(1)-adrenoceptor agonist. However, in 33% of the neurons, NA decreased sIPSC frequency by 54%, and this effect was blocked by yohimbine (2-20 microM), the alpha(2)-adrenoceptor antagonist and mimicked by clonidine (50 microM), the alpha(2)-adrenoceptor agonist. The Na(+) channel blocker, tetrodotoxin (0.1 microM) blocked the alpha(1)-adrenoceptor-mediated effect, but not the alpha(2)-adreonoceptor-mediated one. Both of the stimulatory and inhibitory effects of NA on sIPSC frequency were observed in individual neurons when tested with NA alone, or both phenylephrine and clonidine. Furthermore, in most neurons that showed the stimulatory effects, the inhibitory effects of NA were unmasked after blocking the stimulatory effects by prazosin or tetrodotoxin. These data indicate that tonic GABAergic inputs to the majority of type II PVN neurons are under a dual noradrenergic modulation, the increase in sIPSC frequency via somatic or dendritic alpha(1)-adrenoceptors and the decrease in sIPSC frequency via axonal terminal alpha(2)-adrenoceptors on the presynaptic GABAergic neurons.

Repeated cocaine treatment activates flank marking in adolescent female hamsters

Cocaine abuse during adolescence represents a significant health risk due to the potential for both acute and long-term negative physical and psychological sequelae, including increased aggressive behavior. This study examined the effect of adolescent cocaine treatment on flank marking (i.e., a stereotypic motor behavior that is part of the response pattern of offensive aggression) in female and male Syrian hamsters (Mesocricetus auratus). Adolescent cocaine treatment activated flank marking in female hamsters when animals were measured upon return to their home cage immediately following drug treatment. Sex differences were observed in cocaine-induced flank marking, as males failed to flank mark when returned to the home cage. In females, the behavioral response was most marked on Day 11 of cocaine treatment in all doses tested. Yet, animals treated with low-dose cocaine (0.5 mg/kg/day) showed the most significant increase in flank marking on and from Day 11 forward as compared to medium- and high-dose cocaine-treated animals and controls. In addition, the response of cocaine-treated animals was vigorous and nearly immediate, as >75% of the flank marks scored were performed within the first 2 min of the behavioral test in >85% of animals examined. Measures of locomotion showed that cocaine had stimulatory effects on motor activity in adolescent female hamsters at all doses tested. Cocaine-treated animals did not differ in body weight gain from controls, suggesting no dramatic physiological effects of adolescent cocaine exposure on body growth at the doses tested.

Uterine contractile activity stimulates supraoptic neurons in term pregnant rats via a noradrenergic pathway

Oxytocin secretion is important for the normal progress of parturition in the rat. We tested the hypotheses that contractions of the uterus before pup delivery activate oxytocin neurons, and that they do so via a noradrenergic projection. In anesthetized 22-day (term) pregnant rats, i.v. oxytocin pulses enhanced both uterine contractile activity and the firing rate of oxytocin and vasopressin neurons in the supraoptic nucleus, and these were significantly correlated. The same oxytocin treatment also increased the expression of Fos in both the supraoptic nucleus and the nucleus of the tractus solitarius, but not in 21-day pregnant or virgin rats. In five of eight rats on the day of expected parturition, noradrenaline release in the supraoptic nucleus (sampled by microdialysis) exhibited sudden peaks during oxytocin administration, seen in only one of nine rats given vehicle pulses. Noradrenaline release was significantly greater in rats that went into labor or gave birth to a pup than in rats not in labor. In rats infused with the alpha(1)-noradrenergic receptor antagonist, benoxathian, into the supraoptic nucleus before and during iv oxytocin administration, Fos expression in supraoptic neurons was significantly less than that in vehicle controls. Thus, at term pregnancy, uterine contractions activate both oxytocin and vasopressin neurons in the SON, and this activation involves a noradrenergic pathway.

Norepinephrine and the control of food intake

The focus of the present review is to reconsider the role of endogenous norepinephrine (NE) in brain, specifically within the hypothalamic paraventricular nucleus (PVN), with regard to its potential role in eliciting eating or satiety. The PVN is innervated by NE fibers and is a site at which infusion of exogenous NE elicits eating at low doses. Two subtypes of alpha-adrenergic receptors within the PVN exert antagonistic actions on eating in the rat: activation of PVN alpha(2)-adrenoceptors increases eating, whereas activation of PVN alpha(1)-adrenoceptors suppresses eating. Pharmacologic manipulations that elevate NE can increase or decrease food intake, depending on the site and type of NE manipulation. Certain antiobesity drugs may act to reduce eating via release of NE and subsequent activation of alpha(1)-adrenoceptors. The PVN exhibits a reliable rhythm in the secretion of endogenous NE over the dark-and-light cycle, and this rhythm may interact with changes in numbers of PVN alpha(1)- and alpha(2)-adrenoceptors to modulate eating during the dark-and-light cycle. Push-and-pull and microdialysis studies indicate that NE secretion is strongly associated with eating, particularly at the start of the dark phase. The present review considers potential interactions of NE with substances such as leptin and neuropeptide Y that alter eating.

Noradrenergic regulation of parvocellular neurons in the rat hypothalamic paraventricular nucleus

Noradrenergic projections to the hypothalamic paraventricular nucleus have been implicated in the secretory regulation of several anterior pituitary hormones, including adrenocorticotropin, thyroid-stimulating hormone, growth hormone and prolactin. In an attempt to elucidate the effects of norepinephrine on the central control of pituitary hormone secretion, we looked at the actions of norepinephrine on the electrical properties of putative parvocellular neurons of the paraventricular nucleus using whole-cell current-clamp recordings in hypothalamic slices. About half (51%) of the putative parvocellular neurons recorded responded to norepinephrine with either a synaptic excitation or a direct inhibition. Norepinephrine (30-300microM) caused a marked increase in the frequency of excitatory postsynaptic potentials in about 36% of the parvocellular neurons recorded. The increase in excitatory postsynaptic potentials was blocked by prazosin (10microM), but not by propranolol (10microM) or timolol (20microM), indicating that it was mediated by alpha(1)-adrenoreceptor activation. It was also blocked by ionotropic glutamate receptor antagonists, suggesting that the excitatory postsynaptic potentials were caused by glutamate release. The increase in excitatory postsynaptic potentials was completely abolished by tetrodotoxin, indicating the spike dependence of the norepinephrine-induced glutamate release. In a separate group comprising 14% of the parvocellular neurons recorded, norepinephrine elicited a hyperpolarization (6.2+/-0.69mV) that was blocked by the beta-adrenoreceptor antagonists, propranolol (10microM) and timolol (20microM), but not by the alpha(1)-receptor antagonist, prazosin (10microM). This response was not blocked by tetrodotoxin (1.5-3microM), suggesting that it was caused by a direct postsynaptic action of norepinephrine. The topographic distribution within the paraventricular nucleus of the norepinephrine-responsive and non-responsive parvocellular neurons was mapped based on intracellular biocytin labeling and neurophysin immunohistochemistry. These data indicate that one parvocellular subpopulation, consisting of about 36% of the paraventricular parvocellular neurons, receives an excitatory input from norepinephrine-sensitive local glutamatergic interneurons, while a second, separate subpopulation, representing about 14% of the parvocellular neurons in the paraventricular nucleus, responds directly to norepinephrine with a beta-adrenoreceptor-mediated inhibition. This suggests that excitatory inputs to parvocellular neurons of the paraventricular nucleus are mediated mainly by an intrahypothalamic glutamatergic relay, and that only a relatively small subset of paraventricular parvocellular neurons receives direct noradrenergic inputs, which are primarily inhibitory.

Inhibition of spontaneous inhibitory postsynaptic currents (IPSC) by noradrenaline in rat supraoptic neurons through presynaptic alpha2-adrenoceptors

It has been shown that noradrenergic activation has great influence on the activities of hypothalamic supraoptic neurons. No direct evidence has been reported on the presynaptic effects of adrenoceptors in the actions of noradrenaline on supraoptic neurons, although postsynaptic mechanisms have been studied extensively. In the present study, we explored presynaptic effects of noradrenaline on the supraoptic neurons by measuring spontaneous inhibitory postsynaptic currents (IPSC) with the whole-cell patch-clamp technique. Noradrenaline reduced the frequency of IPSCs in a dose-dependent (10(-9) to 10(-3) M) and reversible manner. Noradrenaline did not affect the amplitude of IPSCs at concentrations of 10(-9) to 10(-5) M, but reduced the amplitude of IPSCs at high concentrations (10(-4) and 10(-3) M). The inhibitory effects of noradrenaline were mimicked by the alpha2-agonist clonidine (10(-4) M), but not by the alpha1-agonist methoxamine (10(-4) M) nor by the beta-agonist isoproterenol (10(-4) M). Moreover, the inhibitory effects of noradrenaline on IPSCs were blocked by the non-selective alpha antagonist phentolamine (10(-4) M) or the selective alpha2-antagonist yohimbine (10(-4) M), but not by the alpha1-antagonist prazosin (10(-4) M). These results suggest that noradrena-line inhibits release of GABA from the presynaptic GABAergic terminals of the supraoptic neurons by activating presynaptic alpha2-adrenoceptors and such presynaptic mechanisms may play a role in the excitatory control of SON neurons by noradrenergic neurons.

The role of alpha-2 adrenoceptors in the regulation of oxytocin neurones in the suckled rat

The role of alpha-2 adrenoceptors in the milk-ejection reflex was investigated by making electrophysiological recordings from oxytocin neurones in the supraoptic nucleus of urethane-anaesthetised rats. Systemic administration of the alpha-2 adrenoceptor antagonist. Idazoxan (0.5 mg/kg, i.v.), temporarily suppressed OT cell bursting activity, while having no consistent action on basal neuronal activity. Clonidine (25 micrograms/kg, i.v.) caused an immediate increase in the frequency and amplitude of oxytocin cell bursting, coincident with a fall in basal activity. A higher dose of clonidine (50 micrograms/kg, i.v.), inhibited both bursting and basal activity. These results indicate that alpha-2 adrenoceptors are essential for the normal functioning of the milk-ejection reflex and may be involved in the facilitatory and inhibitory regulation of suckling-evoked bursting in oxytocin neurones.

Reduced response of the hypothalamo-pituitary-adrenal axis to alpha1-agonist stimulation during lactation

To determine whether altered noradrenergic activation of the hypothalamo-pituitary-adrenal (HPA) axis contributes to the attenuated neuroendocrine response to stress observed during lactation, the effect of intracerebroventricular injection of the alpha1-agonist methoxamine (100 microg) was compared between virgin and lactating rats. Virgin rats showed significant increases in plasma corticosterone after methoxamine, reaching 317 +/- 44 ng/ml at 10 min and remaining significantly elevated for more than 120 min, but lactating rats showed no significant increase in corticosterone levels. Furthermore, methoxamine induced an increase in paraventricular nucleus (PVN) CRF messenger RNA expression in virgin, but not lactating, animals. Both groups of rats exhibited comparable elevations in plasma PRL after methoxamine treatment. Arginine vasopressin messenger RNA expression within the parvocellular PVN was greater in the lactating animals than in the virgin controls, but methoxamine injection was without further effect. Studies performed on ovariectomized virgin rats and ovariectomized rats receiving estradiol or progesterone replacement failed to reproduce the attenuated HPA responses seen after methoxamine treatment, although methoxamine-induced PRL levels were greatly increased by estradiol, probably arising from an effect on hormone synthesis. In vitro electrophysiological recordings of PVN neurons in hypothalamic slices from proestrous virgin and lactating rats showed that 45-52% of neurons in both groups exhibited excitatory responses to 10(-4) M methoxamine, but there was a differential response to 10(-5) M methoxamine, with PVN neurons from lactating animals failing to show a response. These data show a selective down-regulation of alpha1-mediated activation of the HPA axis in lactating animals. This may contribute to the attenuated stress-induced activation of the HPA axis during lactation.

Adrenergic receptor activation hyperpolarizes the caudal neurosecretory cells of the flounder, Platichthys flesus

The physiological factors that govern activity of the caudal neurosecretory system in teleost fish are poorly understood. Immunocytochemical evidence indicates that the neurosecretory Dahlgren cells are innervated by descending monoaminergic fibres. Using intracellular recording techniques in an isolated preparation of the posterior spinal cord of the flounder (Platichthys flesus) we have demonstrated that superfusion of adrenaline or noradrenaline (10(-7) - 10(-3) M) causes hyperpolarization of Dahlgren cells (up to -30 mV). This hyperpolarization is likely to reflect an inhibitory effect of noradrenergic nerves on the neurosecretory system in vivo, reducing the rate of hormone release. Fluctuations in the input resistance and membrane time constant suggest involvement of a multiplicity of cellular mechanisms, including the opening and closing of populations of ion-selective channels. Superfusion with dopamine (10(-7) - 10(-3) M) had no effect. Superfusion with the beta-adrenoreceptor agonist, isoprenaline, caused hyperpolarization but to a markedly lesser extent than the maximum effect of adrenaline or noradrenaline, suggesting that their effects are mediated, only in part, by a beta-adrenoreceptor subtype. Superfusion of the preparation with a membrane permeable, non-hydrolysable cyclic AMP analogue (8-[4-chlorophenylthio]-cAMP) resulted in a slight hyperpolarization which was accompanied by a small, but significant, increase in input resistance. These data are consistent with at least part of the beta-adrenoreceptor mediated effect involving closure of cAMP-sensitive ion channels. Superfusion with the alpha 1-adrenoreceptor agonist, phenylephrine, had no effect on any electrophysiological parameter studied. However, the alpha 2-adrenoreceptor agonist, clonidine, caused hyperpolarization which again failed to reach the maximum level produced by adrenaline or noradrenaline. Together, these data suggest that the adrenergic inhibition of Dahlgren cell activity is mediated by both alpha 2- and beta-adrenoreceptor subtypes.

Inhibition of supraoptic vasopressin neurones following systemic clonidine

Experiments were undertaken in urethane-anaesthetized rats to investigate the effects of systemic clonidine on the firing of supraoptic vasopressin (VP) neurones, which were identified by their characteristic phasic activity pattern. Injection of clonidine (50 micrograms/kg i.v.) reduced the firing of all 16 VP neurones tested, and their overall mean activity decreased from 5.09 +/- 1.01 to 1.63 +/- 0.64 spikes/second (P < 0.02). In VP cells which were already firing phasically before clonidine, the inhibition resulted in complete quiescence. In VP cells which were originally continuously active, the inhibition resulted in a switch to phasic activity. This inhibitory effect, which was prevented by prior injection of the alpha-2 antagonist idazoxan (0.5 mg/kg), had a mean duration of 11.8 +/- 1.8 min. Subsequent experiments revealed that i.v. clonidine (50 micrograms/kg) caused a transient rise in blood pressure, but this had a shorter time-course and was unlikely to account for the prolonged neuronal inhibition. It was concluded that systemic clonidine acts centrally to suppress the activity of hypothalamic VP neurones, thereby explaining the fall in plasma VP levels found in previous studies.

Noradrenergic inputs to sleep-related neurons in the preoptic area from the locus coeruleus and the ventrolateral medulla in the rat

Responses of sleep-related neurons in the preoptic area (POA) to stimulation of the locus coeruleus (LC) and the ventrolateral medulla (VLM), components of the reticular activating system, were recorded in the unanesthetized, head-restrained rat. Single-pulse stimulation of the LC and the VLM, respectively, inhibited 50% and 54% of 30 sleep-active neurons and excited 47% and 67% of 34 waking-active neurons. The remaining neurons were mostly unaffected. Seventy-three neurons that were not related to a sleep-wake state were mostly (i.e., 73-80%) unresponsive to stimulation. The high incidence of responses by sleep-related neurons suggests that neural inputs from the LC and VLM regulate the hypnogenic mechanisms in the POA. Stimulation of the LC antidromically activated 15% of sleep-active neurons and 11% of waking-active neurons. Thus, some of the sleep-related neurons in the POA may regulate LC neurons. In a later stage of the experiment, we used isoflurane-anesthetized rats that had been used for recording sleep-related neurons. Antagonists for adrenoceptors at a concentration of 10 microM were applied to neurons through a multibarrel micropipette to examine the involvement of noradrenaline in the responses as a neurotransmitter. Application of the alpha 2-blocker, yohimbine, attenuated the inhibitory responses in all 7 neurons tested. The beta-blocker, timolol, and the alpha 1-blocker, prazosin, did not alter any of the inhibitory responses. On the other hand, timolol attenuated the excitatory responses in 4 of 7 neurons, and prazosin attenuated the excitatory responses in 5 of 12 neurons. Yohimbine did not affect the excitatory responses. Thus, the LC and the VLM probably inhibit sleep-active neurons through alpha 2-adrenoceptors and excite waking-active neurons through either beta- or alpha 1-adrenoceptors.

Sodium nitroprusside modulates NMDA response in the rat supraoptic neurons in vitro

The modulatory effects of NO on N-methyl-D-aspartate (NMDA)-induced response in neurons of the supraoptic nucleus (SON) were studied by intracellular recording and radioimmunoassay of cyclic nucleotides using the rat brain slice preparation. Depolarization induced by 100 microM NMDA was reduced by application of 1 to 3 mM of the NO-donors, sodium nitroprusside, and isosorbide dinitrate in all 8 neurons and in 6 of 10 neurons, respectively. The scavenger for NO, hemoglobin, and the inhibitor of NO synthase, NG-nitro-L-arginine (LNNA) enhanced the NMDA-induced depolarization in four neurons and two of three neurons, respectively. Intracellular cGMP accumulation induced by NMDA was significantly diminished by LNNA. However, NMDA-induced depolarization was not affected by either the protein kinase inhibitor, N-[2-(methylamino)ethyl]-5- isoquinolinesulfonamide dihydrochloride (H-8), or the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). These results indicate that NO reduces NMDA-induced depolarization in a manner that is independent of cGMP and may control the activity of the SON neurons through NMDA receptors.

Alterations in noradrenergic physiological characteristics with DOCA-hypertension: interaction between norepinephrine and GABA in rat lateral hypothalamus

The lateral hypothalamus (LH) is involved in the central integration of fluid and electrolyte balance. Several studies have suggested a role for norepinephrine (NE) in these functions. In previous studies we presented evidence in support of a modulatory role for NE within the LH circuitry. Specifically, NE facilitated responses of LH cells to synaptic inputs and putative transmitters. In the present studies, we examined the influence of NE on the response of LH neurons to the inhibitory amino acid transmitter GABA. Neuronal responses were studied in normal, DOCA hypertensive, and 1% NaCl diet (HSD)-treated rats. Male rats were uninephrectomized and received a DOCA implant (200 mg/kg). They were given 1% NaCl and 0.1% KCl in their drinking water (4-6 weeks). HSD rats received the same treatment, except that no DOCA was given. Extracellularly recorded responses from single LH neurons to iontophoretic pulses (5-50 nA; 10 s duration) of GABA were examined before, during and after NE microiontophoresis (5-50 nA) in anesthetized rats. The results indicated a shift of NE modulatory action from potentiating to antagonizing GABA-induced inhibition. In control rats, NE routinely potentiated GABA depressant responses (19 of 26, 73%), whereas in HSD rats the ability of NE to enhance GABA responses was reduced to 33% of the cases tested (10 of 30). Likewise, NE did not augment, but rather antagonized GABA inhibition in the majority of cells recorded (21 of 35, 60%) from DOCA hypertensive rats. The beta agonist isoproterenol was still capable of potentiating GABA inhibition of LH cells in HSD and DOCA treated animals, suggesting that the change in the capacity of NE to enhance GABA action is not a result of alterations in beta receptor function, but could arise from a modification of the ratio between alpha- and beta-adrenoceptors. NE modulating capability was also altered-in LH neurons responsive to experimentally induced changes in blood pressure. In summary, these findings suggest that chronic HSD and DOCA treatments can alter the modulatory capacities of NE within the LH. These alterations in noradrenergic action within hypothalamic cardiovascular centers might affect the way neurons respond to afferent baroreceptor information, as well as the way they control sympathetic and parasympathetic effector mechanisms. A decrease in the inhibitory capacities of GABA transmission in these areas, due to alterations of NE, may play a role in the genesis of hypertension.

CCK-8 excites oxytocin-secreting neurons in the paraventricular nucleus in rats--possible involvement of noradrenergic pathway

Systemic administration of CCK-8 increased plasma oxytocin (OXT) level in rats anesthetized with a mixture of urethane and alpha-chloralose. Extracellular recordings were made from magnocellular neurosecretory neurons in the paraventricular nucleus (PVN) of the hypothalamus in anesthetized rats to examine the effects of CCK-8 on the firing of PVN neurons. Thirteen out of 16 nonphasic neurons (putative OXT-secreting neurons) were excited by intravenous and/or intraperitoneal administration of CCK-8. By contrast, 8 out of 10 phasic cells, vasopressin(AVP)-secreting cells, were inhibited by systemic administration of CCK-8. Four out of five nonneurosecretory neurons in the PVN were excited by the administration of CCK-8. Moreover, microiontophoretically applied phentolamine blocked the excitatory responses induced by CCK-8 in nonphasic neurons. We measured extracellular noradrenaline (NA) level in the PVN, using in vivo microdialysis. Intravenous administration of CCK-8 induced NA release in the PVN. These results suggest that CCK-8 activates the excitatory afferent pathway to putative OXT-secreting neurons in the PVN which may, at least in part, be involved in the central noradrenergic projection.

Modulation of feeding by hypothalamic paraventricular nucleus alpha 1- and alpha 2-adrenergic receptors

Noradrenergic receptor populations within the paraventricular hypothalamus (PVN) modulate feeding. Satiated rats exhibit enhanced feeding subsequent to activation of alpha 2-adrenergic receptors within the PVN induced by exogenous infusion of either norepinephrine (NE) or clonidine (CLON). The feeding-stimulatory effect of alpha 2-adrenergic agents presumably reflects an inhibitory action on receptors located on medial hypothalamic "satiety" cells. Adrenergic receptors of the alpha 1-subclass have been identified within the PVN which are excitatory and which may function to suppress food intake. Microinjection into rat PVN of various alpha 1-adrenergic agonists including cirazoline, methoxamine, phenylpropanolamine and phenylephrine suppress feeding; an effect that is reversed by pretreatment with alpha 1-adrenergic receptor antagonists. The present review argues that alpha 1- and alpha 2-adrenoceptors within brain and specifically within the PVN are organized in an antagonistic fashion and that the effects of various adrenergic agonists on feeding may reflect the degree to which these agonists act at alpha 1- and alpha 2-adrenoceptors as well the relative balance of these receptors and their activity within the PVN.

Norepinephrine inhibits vasopressin-stimulated flank marking in the Syrian hamster by acting within the medial preoptic-anterior hypothalamus

Syrian hamsters exhibit a form of scent marking behavior called flank marking. Flank marking, which is elicited during social contact with other hamsters and by the odors of other hamsters, communicates socially important information such as mate choice and dominance status. Vasopressinergic activity within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) is essential for the expression of flank marking. In female hamsters, an inverse relationship exists between the expression of flank marking and sexual receptivity during the 4-day estrous cycle. Since norepinephrine (NE) appears to facilitate sexual receptivity, the present study investigated whether NE might have an inhibitory effect on flank marking by acting on Vasopressinergic activity within the MPOA-AH. Microinjection of 9.0 μM arginine vasopressin (AVP) into the MPOA-AH stimulated high levels of flank marking. Microinjection of 9.0 μM AVP combined with NE in concentrations of 4.0, 0.4 or 0.2 nM, drastically reduced or eliminated flank marking. In contrast, AVP in combination with 0.09, 0.04 or 0.004 nM NE produced no significant reductions in flank marking. In addition, microinjection of 9.0 μM AVP, in combination with epinephrine (4.0 nM), but not dopamine (4.0 nM), serotonin (4.0 nM) or neuropeptide Y (900 μM), significantly reduced AVP-induced flank marking. In male hamsters, microinjection of NE (4.0 nM) combined with AVP (9.0 μM) into the MPOA-AH was not found to inhibit AVP-stimulated flank marking. These results suggest that NE is involved in regulating the expression of flank marking during the estrous cycle by acting on Vasopressinergic activity within the MPOA-AH.

Osmotic stimulation induces changes in the expression of beta-adrenergic receptors and nuclear volume of astrocytes in supraoptic nucleus of the rat

The influence of osmotic stimulation on the density of beta-adrenoceptor binding sites in the rat supraoptic nucleus (SON) was studied by quantitative autoradiography using 125I-cyanopindolol (ICYP). Increased density of beta-adrenoceptor binding sites was observed in osmotically stimulated rats and also after the suppression of neuronal activation by rehydration of animals. This was mainly due to a significant increase in the concentration of beta 2 binding sites. The overexpression of beta-adrenoceptors occurred concomitantly with nuclear expansion in SON astrocytes. Moreover, the higher concentration of beta-adrenoceptors observed in the ventral portion of the SON largely coincided with the area that showed intense GFAP-immunostaining. These results provide indirect evidence of an astrocytic location of beta-adrenoceptors and also of beta-adrenergic mediation in the structural and functional changes of SON astrocytes.

Acute effect of an alpha1-adrenoceptor antagonist on urinary sodium excretion, plasma atrial natriuretic peptide, arginine vasopressin, and the renin-aldosterone system in healthy subjects

To elucidate the mechanism underlying the sodium retention caused by alpha 1-adrenoceptor blockade in man, a placebo-controlled, randomised, double-blind study has been made of the acute effects of bunazosin an alpha 1-antagonist, on urinary sodium excretion, atrial natriuretic peptide (ANP), arginine vasopressin (AVP), and the renin-aldosterone system in 7 healthy men. A single oral dose of bunazosin 2.0 mg caused a significant reduction (P less than 0.05) in urinary sodium excretion after 0-2 h, 2-4 h, and 4-6 h. The mean values for plasma ANP, AVP, aldosterone, and cortisol concentrations at those times were similar after placebo and bunazosin, and plasma renin activity was significantly increased 2 and 4 h after bunazosin. Pretreatment with oral enalapril 10 mg, an angiotensin converting enzyme inhibitor, did not prevent the bunazosin-induced reduction in urinary sodium excretion. There was a significant positive correlation between the drug-induced changes in blood pressure and urinary sodium excretion. The results suggest that ANP, AVP, and renin-aldosterone may play little role in the sodium retention caused by acute alpha 1-adrenoceptor blockade in man.

Expression of Fos-like immunoreactivity by yohimbine and clonidine in the rat brain

To elucidate the role of alpha 2-adrenoceptors in transcriptional control in the rat brain, we localized the Fos-like immunoreactivity (Fos-LI) induced by alpha 2-adrenoceptor agonists and by an antagonist. Injections of yohimbine (5 mg/kg, i.p.) into rats led to the induction of Fos-LI in areas with a dense alpha 2-adrenoceptor binding such as the locus coeruleus, the bed nucleus of stria terminalis, the central nucleus of amygdaloid complex, the paraventricular nucleus, the nucleus tractus solitarius, and ventrolateral medulla oblongata. Clonidine (500 micrograms/kg, i.p.) suppressed the Fos expression by yohimbine in these nuclei, and clonidine (100 micrograms/kg, i.p.) or guanabenz (4 mg/kg, i.p.) induced Fos-LI in oxytocin neurons in the paraventricular and supraoptic nuclei in the hypothalamus. Thus, the alpha 2-adrenoceptor is involved in transcriptional control via Fos expression in neurons related to autonomic and other functions.

Mapping study of noradrenergic stimulation of vasopressin release

The precise role of hypothalamic norepinephrine (NE) in the control of vasopressin (AVP) release has remained unclear, due to reports of both inhibitory and excitatory effects of NE and only a few studies with direct hypothalamic manipulations. The present study utilized a chronically implanted swivel brain cannula to investigate, in undisturbed and freely behaving rats, the impact of acute hypothalamic infusions of monoamines on circulating AVP levels. The first study examined and compared the responsiveness of six hypothalamic sites to NE infusion through the swivel cannula. Results indicated that the excitatory effect of central noradrenergic stimulation on serum AVP is highly site specific, localized to the paraventricular (PVN) and supraoptic (SON) nuclei. These two nuclei appeared to be equally responsive to NE infusion, yielding a threefold rise in serum AVP over baseline levels. In contrast, NE in the dorsomedial nucleus produced a significantly smaller increase in AVP, and no response was observed in the ventromedial nucleus, posterior hypothalamus, or perifornical lateral hypothalamus. Further tests conducted in the PVN showed this nucleus to respond in a dose-dependent manner to NE infusion. In contrast, under similar test conditions, dopamine caused only a small increase in AVP at a relatively high dose, while a PVN injection of serotonin produced no response. These results support the existence of an excitatory noradrenergic system controlling AVP release and specifically demonstrate that this function of NE is localized to the PVN and SON, in contrast to other hypothalamic areas, and is mimicked to some extent by dopamine but not by serotonin.

Kappa-selective opioid receptor agonists leumorphin and dynorphin inhibit supraoptic neurons in rat hypothalamic slice preparations

Abstract To clarify the effects of opioid peptides, and in particular the effects of kappa-receptor agonists on the activity of supraoptic neurons, extracellular recordings were made from 71 spontaneously firing neurons in the rat hypothalamic slice preparation. Of 71 neurons, 28 showed a phasic firing pattern (phasic neurons: putative vasopressin neurons). The mean firing rate of phasic neurons was 2.6 spikes/s (intraburst firing rate 5.4 +/- 2.2 spikes/s). The mean firing rate of neurons classified as non-phasic neurons (putative oxytocin neurons) was 4.5 spikes/s. Following bath application of leumorphin (LM) at 10(-7) M, which has potent opioid activity at kappa-receptors, 17 (61%) of 28 phasic neurons were inhibited and 22 (51%) of 43 non-phasic neurons were inhibited. Excitation was observed in only one non-phasic neuron. The dose-dependence of the response to LM was tested in five supraoptic neurons. There was an inverse relationship between LM concentration and percent change in firing rate. The threshold concentration of LM was approximately 10(-8) M. The relatively selective kappa-receptor antagonist, MR-2266, completely blocked the LM-induced responses. Its effects were long-lasting and only partial recovery was observed 2 h after the application of MR-2266. Dynorphin had similar inhibitory effects on supraoptic neurons to those obtained with LM when tested on the same neurons. In another series of experiments the mu-receptor agonist morphine and the delta-receptor agonist [D-Ala, D-leu]-enkephalin (DADLE) were applied to 28 supraoptic neurons (12 phasic and 16 non-phasic neurons) at 10(-7) M and their actions compared directly with that of LM. Only two of 12 phasic neurons tested were inhibited by DADLE and none of five phasic neurons tested was inhibited by morphine, while eight of the 12 neurons tested were inhibited by LM. By contrast the non-phasic neurons tested were inhibited by application of each of the peptides; seven of 16 neurons tested were not only inhibited by LM, but also five of 11 neurons by DADLE and seven of 15 by morphine. The magnitude of the responses varied from cell to cell. These results suggest that LM acts at the same receptors as dynorphin, and that opioids acting preferentially at kappa-receptors inhibit both vasopressin and oxytocin neurons while delta- and mu-receptor agonists inhibit primarily oxytocin neurons.

Excitation of supraoptic vasopressin cells by stimulation of the A1 noradrenaline cell group: failure to demonstrate role for established adrenergic or amino acid receptors

The effects of adrenergic and excitatory amino acid antagonists on supraoptic nucleus (SON) neurosecretory cell responses to stimulation of the A1 noradrenaline (NA) cell group were examined in anaesthetized male rats. As in previous studies, delivery of cathodal pulses (100 microA, 1 ms pulses, 1 Hz) to the A1 region of the caudal ventrolateral medulla excited spontaneously active, antidromically identified neurosecretory cells, the majority of which were identified as arginine vasopressin (AVP) secreting on the basis of basal discharge patterns and responses to abrupt increases in arterial blood pressure. Administration of alpha- and beta-adrenoreceptor antagonists, by systemic or intracerebroventricular delivery of a bolus, or by direct pressure injection into the SON, did not alter neurosecretory cell responses to A1 stimulation, even when doses applied exceeded that required for blockade of excitations elicited by local application of NA. Application of the broad spectrum excitatory amino acid antagonist kynurenic acid (5-40 mM) blocked the excitatory effects of locally applied glutamate (100 microM) and transiently inhibited spontaneous activity, but failed to alter the excitatory effects of A1 region stimulation on SON cells. Identical effects were obtained with a selective kainate/quisqualate receptor antagonist. These data indicate that neurosecretory cell responses to activation of the A1 cell group are unaltered by antagonists of alpha- and beta-adrenoreceptors, or excitatory amino acid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent connections of the rat's supraoptic nucleus

Neurons projecting to the supraoptic nucleus (SON) have been identified following stereotaxic injections of either horseradish peroxidase or fast blue into the SON region of adult rats. The subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis and medial septal nucleus were the source of the largest numbers of supraoptic-projecting neurons. Several smaller projections also originate from the ipsilateral locus coeruleus, preoptic area, lateral parolfactorial area, dorsomedial nucleus of the hypothalamus, lateral parabrachial nucleus and ventrolateral medulla. Several other areas appeared to project only to the region immediately dorsal to the SON: lateral septal nucleus, diagonal band of Broca, ventral tegmental nucleus, and the supramamillary nucleus. These areas may influence SON neurosecretory function by way of interneurons found immediately dorsal to SON. Additional areas were identified with retrograde fluorescent label only, and these projected to the area immediately dorsal to SON and/or to SON itself.

Stimulation of the rostral ventrolateral medullary neurons increases cortical cerebral blood flow via activation of the intracerebral neural pathway

In urethane-anesthetized, artificially ventilated rats, electrical or chemical (by L-glutamate) focal stimulations of the rostral ventrolateral medulla (RVLM) produced an increase in cortical cerebral blood flow (CBF). The RVLM-induced cortical vasodilative response was present in animals with spinal cords sectioned at levels of Th3-4 and with bilateral extracerebral cervical sympathetic trunks (CSTs) severed. The RVLM-induced cortical vasodilative response was totally eliminated by an alpha 2 adrenergic blocker, but not by blockers for muscarinic, nicotinic, alpha 1 and beta receptors. It was concluded that there is an intracerebral vasodilative neural pathway including an alpha 2 adrenergic receptor originating in the RVLM for regulation of cortical blood vessels.

Neuropeptide Y potentiates excitation of supraoptic neurosecretory cells by noradrenaline

The effects of neuropeptide Y (NPY) and noradrenaline (NA) on the activity of rat supraoptic nucleus (SON) neurosecretory cells were examined using perfused hypothalamic slices. Bath application of either NPY (10(-9)-10(-6) M) or NA (10(-6)-10(-3) M) excited SON cells, although only NA elicited consistent, dose-dependent effects. Application of NPY at a dose having virtually no direct effects (10(-8) M) produced a 5-fold increase in SON cell responsiveness to NA at the sub-maximal response dose of 10(-5) M, but did not alter the minimum concentration of NA required to excite SON cells or increase the maximal response elicited by higher NA concentrations. The effects of NA, alone or in combination with NPY, were abolished by alpha-adrenoreceptor blockade. These data suggest that NPY has only weak direct effects on neurosecretory cells, but may have important neuromodulatory actions, significantly enhancing the excitatory effects of NA.

Responses of hypothalamic paraventricular neurons in vitro to norepinephrine and other feeding-relevant agents

To investigate paraventricular hypothalamic neuronal actions responsible for the effects of neurotransmitters on feeding, and to test the notion that a single population of cells there could account for feeding effects, hypothalamic slices containing the paraventricular nucleus (PVN) were prepared from rats. Electrophysiological responses of individual PVN neurons to feeding-inducing agents norepinephrine (NE) and gamma-aminobutyric acid (GABA), and to anorexic agents serotonin (5-HT) and histamine (Hist) were examined. NE inhibited neuronal activity through alpha 2-adrenergic receptors, and excited through alpha 1-receptors. alpha 2-receptors are known to mediate the behavioral effect of NE. NE inhibited most clearly those neurons that otherwise fired continuously in this type of in vitro preparation. GABA affected the activity of 37% of the neurons tested, primarily by inhibition. The inhibitory action of GABA can be related to its feeding-inducing effect. GABA in PVN can also attenuate excitatory responses and enhance inhibitory responses to NE or 5-HT. 5-HT caused excitatory and inhibitory responses with the former action outnumbering the latter by approximately 3 to 1. Since this would result in a net excitation, it appears that 5-HT in PVN inhibits feeding mainly by exciting neuronal activity. Hist excited 72% and inhibited only 2% of PVN neurons. The excitation was blocked by H1-antagonists, which have been shown to mediate Hist effect on feeding. Comparing across neurons, the inhibitory response to NE was correlated with that to GABA, but not with any responses to 5-HT or Hist. The excitatory responses to Hist correlated with 5-HT responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of caudal ventrolateral medulla neurons in mediating visceroreceptive information to the hypothalamic paraventricular nucleus

Both neurohypophyseal and tuberoinfundibular neurosecretory neurons in the PVN received excitatory synaptic inputs from the CVLM. We electrophysiologically identified neurons in the CVLM which project to the PVN. On the basis of antidromic spike latencies, two different populations of neurons could be differentiated: slow- and fast-conducting cells. Slow-conducting cells which were presumed to be A1 catecholaminergic cells, received inhibitory and excitatory synaptic inputs from arterial baroreceptors and the cervical vagus nerve, respectively. Our results suggest that slow conducting cells in the CVLM which cause excitation of PVN neurons via a monosynaptic pathway, mediate visceroreceptive information to the PVN.

Neuronal plasticity in the hedgehog supraoptic nucleus during hibernation

The purpose of the present study was to identify processes of plasticity in the receptive field of neurosecretory neurons of the supraoptic nucleus during hibernation in the hedgehog, in order to correlate them with the increased neurosecretory activity observed in this nucleus during this annual period. Using the Rapid Golgi method, a quantitative study was conducted in the receptive field of bipolar and multipolar neurons (the main components of the nucleus). Results indicate a generalized increase in the following characteristics: (1) number of dendritic spines per millimeter along the dendritic shafts; (2) degree of branching in the dendritic field; and (3) dendritic density around the neuronal soma. These data demonstrate modification of the dendritic field in the supraoptic nucleus during hibernation, a change undoubtedly related to functional conditions. Since the observed changes affect structures such as dendritic spines which are directly related to the arrival of neural afferences, the discussion is centered on the types of stimuli which may be responsible for the observed processes.

Cortisol suppresses noradrenaline-induced excitatory responses of neurons in the paraventricular nucleus; an in vitro study

The effects of cortisol on noradrenaline (NA)-induced responses of neurons in the parvocellular division of the paraventricular nucleus (PVN) were investigated in hypothalamic slices of rats. Cortisol (10(-5) M)-induced excitation in 7 out of 11 PVN neurons tested and inhibited 3 out of 8 PVN neurons which were not affected by NA. Cortisol alone did not affect the basal firing rate of the PVN neurons. We conclude that cortisol may have an inhibitory effect on neurons in the parvocellular division of the PVN through depression of the noradrenergic system.

Direct catecholaminergic projection from nucleus tractus solitarii to supraoptic nucleus

To determine whether the supraoptic nucleus (SON) receives a direct projection from catecholamine cells of the nucleus tractus solitarii (NTS), retrograde transport of rhodamine-tagged latex microspheres was combined with a procedure for the fluorescence histochemical visualization of catecholamines. SON tracer injections, made via transpharyngeal approach, retrogradely labelled cells at all levels of NTS, although the majority were located caudal to obex with an ipsilateral predominance. Approximately half of these cells were also identified as catecholaminergic; the relatively caudal level in the dorsomedial medulla of most of these cells suggests that they probably correspond to the A2 catecholamine cell group.

Hypothalamic paraventricular nucleus: interaction between alpha 2-noradrenergic system and circulating hormones and nutrients in relation to energy balance

Extensive evidence suggests that norepinephrine (NE) in the brain is active in the control of eating behavior. Central injection studies demonstrate a stimulatory effect of NE on food intake, a response which is mediated by alpha 2-noradrenergic receptors located in the medial hypothalamus, in particular the paraventricular nucleus (PVN). Activation of these PVN receptors stimulates ingestion specifically of carbohydrate-rich foods, and this response is believed to reflect the role of endogenous NE in controlling natural appetite for this macronutrient. This alpha 2-noradrenergic system in the PVN appears to be physiologically activated at the onset of the animals' active cycle, when there is a natural peak in preference for carbohydrate. At this time, the adrenal hormone corticosterone, which is known to play a major role in carbohydrate metabolism, is found to interact positively with NE in the potentiation of carbohydrate ingestion. Circulating glucose also influences the activity of PVN alpha 2-noradrenergic receptors at this time, and, moreover, alpha-noradrenergic stimulation of the PVN produces an increase in circulating levels of both corticosterone and glucose. This and other evidence has led to the hypothesis that NE in the PVN, through the activation of glucocorticoid- and glucose-sensitive alpha 2-receptor sites, is physiologically active in energy homeostasis, most particularly at the onset of the animal's active cycle. Specifically, this neurotransmitter in the PVN evokes a state of energy conservation. This state involves adjustments in carbohydrate ingestion as well as metabolism, that allow animals to maintain energy reserves by anticipating or responding to a depletion.

Thermal, osmotic and chemical modulation of neural activity in the paraventricular nucleus: in vitro studies

To investigate the functions of the paraventricular nucleus (PVN) which plays an important role as an integration site for the neuroendocrine and autonomic nervous systems, the firing activity of PVN neurons was recorded from hypothalamic slice preparations during thermal, osmotic and chemical stimulation. Neurons responded to environmental factors such as temperature and osmolarity and both warm-responsive and cold-responsive neurons were observed in the PVN. Some PVN neurons were also osmoresponsive and unlike neurons in the supraoptic nucleus, most osmoresponsive PVN neurons decreased their firing rate during hyperosmotic stimulation. One of the classical transmitters, noradrenaline, exerted excitatory effects on PVN neurons through alpha 1- and beta-receptors and inhibitory responses through alpha 2-receptors. Atrial natriuretic polypeptide exerted inhibitory effects on putative parvocellular PVN neurons but it had no effect on putative magnocellular PVN neurons. An endogenous sugar derivative, 2-deoxytetronic acid, thought to be an endogenous satiety factor, elicited inhibitory effects, supporting the possibility that the PVN also may be related to feeding behaviour. Arginine-vasopressin and oxytocin which are synthesised in the magnocellular neurosecretory cells excited PVN neurons, suggesting that the PVN may have short circuits modulating neural activity within the nucleus itself. We conclude that neurons in the PVN may receive multiple information and act as one of the important integrative sites in the brain.

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters

The present studies were conducted as part of an ongoing investigation of the effects of norepinephrine (NE) in neuronal circuits of the mammalian brain. In this report, we describe noradrenergic actions in the lateral hypothalamus (LH), an area which has been implicated in the central integration of cardiovascular regulatory mechanisms, fluid balance and ingestive behaviors. Microiontophoretically applied NE was interacted with extracellularly recorded responses of LH neurons to iontophoretically applied putative neurotransmitters gamma-aminobutyric acid (GABA), acetylcholine (ACh) and glutamate (Glu); and activation of known input pathways from the reticular thalamus (RT) and the lateral preoptic area (LPO). Peri-event histograms of cell responses were computed before, during and after NE microiontophoresis (5-50 nA) and used to quantitatively evaluate monoamine-induced effects on spontaneous and stimulus evoked activity of LH neurons. In 16 of 23 LH neurons, RT-stimulus-induced inhibition was markedly prolonged from a mean of 28.3 +/- 4.8 ms to 44.7 +/- 5.2 ms, during iontophoretic application of NE. In 22 of 38 LH cells, LPO-stimulus-induced excitatory responses were enhanced above control levels during NE administration. In further tests, inhibitory responses of LH cells to iontophoretic pulses of GABA were potentiated during NE administration in 69% (24 of 35) of the cases tested. ACh-induced excitation was potentiated in 9 of 21 cells. In 4 of these cases, otherwise subthreshold doses of ACh caused marked increases in cell firing during the period of NE administration. By contrast, Glu-evoked excitation was antagonized by NE iontophoresis in 65.5% (17 of 26) of LH cells tested. These findings indicate that, as in other noradrenergic target regions of the CNS, NE can facilitate synaptically mediated responses of LH neurons. Taken together these observations suggest that NE may play an important regulatory role in the synaptic transfer of information within LH circuits, and consequently exert considerable influence over the homeostatic functions mediated by this structure.

Inhibitory effects of 2-deoxytetronic acid, a putative endogenous satiety factor, on paraventricular neurons of rats, in vitro

The endogenous sugar acid, 2-deoxytetronic acid (2-DTA) was investigated for its effects on activity of paraventricular nucleus (PVN) neurons in slice preparations from rats. 2-DTA dose-responsively inhibited the spontaneous activity of 23 of the 53 PVN neurons tested at concentrations of 10(-5) to 10(-2) M, and 2 were excited. The inhibitory responses were not due to osmotic changes by 2-DTA administration. The high concentration of 2-DTA needed to produce responses suggests that it acts as substrate for oxidative metabolism rather than as a neurotransmitter or neuromodulator in the suppression of feeding behavior.

Oxytocin predominantly excites putative oxytocin neurons in the rat supraoptic nucleus in vitro

To determine the oxytocin (OXT) sensitivity of neurons in the supraoptic nucleus (SON), extracellular recordings were made from the rat hypothalamic slice preparation. OXT added to the bathing medium (3 X 10(-7) M) excited 13 (93%) of 14 cells which fired continuously (average 4.9 +/- 0.7 spikes/s) and 26 (81%) of 32 cells which fired slowly and irregularly (average 1.4 +/- 0.4 spikes/s). By contrast, only 2 (8%) of 26 phasically firing neurons were excited and none of the SON cells tested were inhibited. The excitation was reversibly antagonized by a synthetic OXT analogue, 1-deamino-[2-(O-methyltyrosine), 4-valine, 8-D-arginine]vasopressin. The results suggest that OXT exerts predominantly excitatory effects in the SON and that putative OXT cells are more likely to be affected than putative vasopressin cells.

Modulation of oxytocin secretion by ascending noradrenergic pathways: sexual dimorphism in rats

We have investigated the role of ascending noradrenergic pathways in the control of oxytocin (OT) and arginine-vasopressin (AVP) secretion during acute immobilization stress in male and female rats. 6-Hydroxydopamine-induced lesions of the ventral noradrenergic bundle (VNAB) resulted in a selective depletion of hypothalamic noradrenaline content. In sham-lesioned rats plasma levels of OT were raised following stress, the response being significantly greater in female compared with male animals. VNAB lesions were not associated with altered responses in female rats, whereas lesioned males exhibited markedly elevated OT stress responses. AVP secretion was not modulated in VNAB-lesioned rats of either sex. The results provide functional evidence of a sexually dimorphic inhibitory role of the VNAB in the control of OT secretion.

Depolarizing effect of noradrenaline on neurons of the rat supraoptic nucleus in vitro

Noradrenaline (NA) (1-100 microM) was applied to 41 neurons recorded intracellularly from the supraoptic nucleus (SON) of the rat hypothalamic slice preparation; 34 (83%) neurons showed membrane depolarization which was dose-dependent. The depolarization was frequently accompanied by decreased membrane resistance, increased firing rate and increased fluctuations in membrane potential. Following the application of the alpha-agonist, phenylephrine, 10 out of 11 neurons tested showed similar responses, while the beta-agonist, isoproterenol, caused no changes in 6 out of 7 SON cells. We found no difference in responsiveness between neurons having a 'phasic' or a 'non-phasic' pattern of firing. We conclude that NA depolarized and increased the firing rate of both vasopressin- and oxytocin-containing neurons through an action on alpha-adrenergic receptors.