Diagonal band projection towards the hypothalamic supraoptic nucleus: light and electron microscopic observations in the rat

Cardiovascular Neuroendocrinology: Emerging Role for Neurohypophyseal Hormones in Pathophysiology

Arginine vasopressin (AVP) and oxytocin (OXY) are released by magnocellular neurosecretory cells that project to the posterior pituitary. While AVP and OXY currently receive more attention for their contributions to affiliative behavior, this mini-review discusses their roles in cardiovascular function broadly defined to include indirect effects that influence cardiovascular function. The traditional view is that neither AVP nor OXY contributes to basal cardiovascular function, although some recent studies suggest that this position might be re-evaluated. More evidence indicates that adaptations and neuroplasticity of AVP and OXY neurons contribute to cardiovascular pathophysiology.

Activation of septal OXTr neurons induces anxiety- but not depressive-like behaviors

The neuropeptide oxytocin (OXT) is well recognized for eliciting anxiolytic effects and promoting social reward. However, emerging evidence shows that OXT increases aversive events. These seemingly inconsistent results may be attributable to the broad OXT receptor (OXTr) expression in the central nervous system. This study selectively activated septal neurons expressing OXTr using chemogenetics. We found that chemogenetic activation of septal OXTr neurons induced anxiety- but not depressive-like behavior. In addition, septal OXTr neurons projected dense fibers to the horizontal diagonal band of Broca (HDB), and selective stimulation of those HDB projections also elicited anxiety-like behaviors. We also found that septal OXTr neurons express the vesicular GABA transporter (vGAT) protein and optogenetic stimulation of septal OXTr projections to the HDB inactivated HDB neurons. Our data collectively reveal that septal OXTr neurons increase anxiety by projecting inhibitory GABAergic inputs to the HDB.

P2X2 Receptor Expression and Function Is Upregulated in the Rat Supraoptic Nucleus Stimulated Through Refeeding After Fasting

Magnocellular neurons in the supraoptic nucleus (SON), which synthesize and release arginine vasopressin (AVP) and oxytocin (OT), express several subtypes of ATP-stimulated purinergic P2X receptors (P2XR) that modulate neuronal activity as well as neurotransmitter and hormone release. However, the physiological impact of this modulation is not well understood. Here, we tested a hypothesis that P2XRs play a role in the sustained release of hormones from SON neurons stimulated through fasting/refeeding. We studied the effect of 2 h of refeeding after 48 h of fasting on P2XR and P2YR mRNA expression and ATP-induced presynaptic and postsynaptic responses in the SON of 30-day-old rats. Quantitative real-time PCR revealed that the expression of P2X2R and AVP mRNA was upregulated, whereas P2X4R, P2X7R, P2Y2R, and OT mRNA levels were not significantly changed and P2Y1R mRNA expression was decreased. Whole-cell patch clamp recordings performed on isolated rat brain slices showed that the amplitude of the ATP-stimulated somatic current and the ATP-induced increases in the frequency of spontaneous GABAergic inhibitory postsynaptic currents were significantly higher in SON neurons from fasted/refed rats than in SON neurons from normally fed rats. No evidence was found for changes in the presynaptic effect of ATP in SON neurons not expressing somatic P2XRs. These results suggest that the increased activity of SON neurons synthesizing AVP is associated with enhanced expression of P2X2Rs on neuronal cell bodies and their GABAergic presynaptic nerve terminals.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

Effects of Salt Loading on the Regulation of Rat Hypothalamic Magnocellular Neurosecretory Cells by Ionotropic GABA and Glycine Receptors

Synaptic and extrasynaptic transmission mediated by ionotropic GABA and glycine receptors plays a critical role in shaping the action potential firing (spiking) activity of hypothalamic magnocellular neurosecretory cells and therefore determines the rate at which vasopressin and oxytocin are released from the neurohypophysis. The inhibitory effect of these transmitters relies on the maintenance of a low concentration of intracellular chloride ions such that, when activated by GABA or glycine, a hyperpolarisation of the neuronal membrane potential results. In this review, we highlight the various ways by which the two types of inhibitory receptors contribute to homeostasis by fine-tuning the spiking rate of vasopressin-releasing magnocellular neurosecretory cells in a manner dependent on the hydration state of the animal. In addition, we review the currently available evidence on how the strength of these inhibitory pathways can be regulated during chronic hypernatraemia via a form of activity-dependent depolarisation of the chloride reversal potential, leading to an abolition of these inhibitory pathways potentially causing sodium-dependent elevations in blood pressure.

Characteristics of GABAergic and cholinergic neurons in perinuclear zone of mouse supraoptic nucleus

The perinuclear zone (PNZ) of the supraoptic nucleus (SON) contains some GABAergic and cholinergic neurons thought to innervate the SON proper. In mice expressing enhanced green fluorescent protein (eGFP) in association with glutamate decarboxylase (GAD)65 we found an abundance of GAD65-eGFP neurons in the PNZ, whereas in mice expressing GAD67-eGFP, there were few labeled PNZ neurons. In mice expressing choline acetyltransferase (ChAT)-eGFP, large, brightly fluorescent and small, dimly fluorescent ChAT-eGFP neurons were present in the PNZ. The small ChAT-eGFP and GAD65-eGFP neurons exhibited a low-threshold depolarizing potential consistent with a low-threshold spike, with little transient outward rectification. Large ChAT-eGFP neurons exhibited strong transient outward rectification and a large hyperpolarizing spike afterpotential, very similar to that of magnocellular vasopressin and oxytocin neurons. Thus the large soma and transient outward rectification of large ChAT-eGFP neurons suggest that these neurons would be difficult to distinguish from magnocellular SON neurons in dissociated preparations by these criteria. Large, but not small, ChAT-eGFP neurons were immunostained with ChAT antibody (AB144p). Reconstructed neurons revealed a few processes encroaching near and passing through the SON from all types but no clear evidence of a terminal axon arbor. Large ChAT-eGFP neurons were usually oriented vertically and had four or five dendrites with multiple branches and an axon with many collaterals and local arborizations. Small ChAT-eGFP neurons had a more restricted dendritic tree compared with parvocellular GAD65 neurons, the latter of which had long thin processes oriented mediolaterally. Thus many of the characteristics found previously in unidentified, small PNZ neurons are also found in identified GABAergic neurons and in a population of smaller ChAT-eGFP neurons.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Role of neuropeptide FF in central cardiovascular and neuroendocrine regulation

Neuropeptide FF (NPFF) is an octapeptide belonging to the RFamide family of peptides that have been implicated in a wide variety of physiological functions in the brain including central cardiovascular and neuroendocrine regulation. The effects of these peptides are mediated via NPFF1 and NPFF2 receptors that are abundantly expressed in the rat and human brain. Herein, we review evidence for the role of NPFF in central regulation of blood pressure particularly within the brainstem and the hypothalamic paraventricular nucleus (PVN). At a cellular level, NPFF demonstrates distinct responses in magnocellular and parvocellular neurons of the PVN, which regulate the secretion of neurohypophyseal hormones and sympathetic outflow, respectively. Finally, the presence of NPFF system in the human brain and its alterations within the hypertensive brain are discussed.

ΔFosB in the supraoptic nucleus contributes to hyponatremia in rats with cirrhosis

Bile duct ligation (BDL), a model of hepatic cirrhosis, is associated with dilutional hyponatremia and inappropriate vasopressin release. ΔFosB staining was significantly increased in vasopressin and oxytocin magnocellular neurosecretory cells in the supraoptic nucleus (SON) of BDL rats. We tested the role of SON ΔFosB in fluid retention following BDL by injecting the SON (n = 10) with 400 nl of an adeno-associated virus (AAV) vector expressing ΔJunD (a dominant negative construct for ΔFosB) plus green fluorescent protein (GFP) (AAV-GFP-ΔJunD). Controls were either noninjected or injected with an AAV vector expressing only GFP. Three weeks after BDL or sham ligation surgery, rats were individually housed in metabolism cages for 1 wk. Average daily water intake was significantly elevated in all BDL rats compared with sham ligated controls. Average daily urine output was significantly greater in AAV-GFP-ΔJunD-treated BDL rats compared with all other groups. Daily average urine sodium concentration was significantly lower in AAV-GFP-ΔJunD-treated BDL rats than the other groups, although average daily sodium excretion was not different among the groups. SON expression of ΔJunD produced a diuresis in BDL rats that may be related to decreased circulating levels of vasopressin or oxytocin. These findings support the view that ΔFosB expression in SON magnocellular secretory cells contribute to dilutional hyponatremia in BDL rats.

Tonic regulation of GABAergic synaptic activity on vasopressin neurones by cannabinoids

Synaptic activity in magnocellular neurosecretory neurones is influenced by the retrograde (i.e. somatodendritic) release of vasopressin, oxytocin and cannabinoids (CBs). For oxytocin neurones, oxytocin exerts constitutive effects on pre-synaptic activity through its ability to release CBs post-synaptically. In the present study, we examined evoked inhibitory post-synaptic currents (eIPSCs) and spontaneous inhibitory post-synaptic currents (sIPSCs) in identified vasopressin (VP) neurones in coronal slices from virgin rats to determine: (i) the extent to which CBs may also tonically modulate VP synaptic activity; and (ii) to determine whether depolarisation-induced suppression of inhibition was present in VP neurones, and if so, whether it was mediated by VP or CBs. The CB1 antagonists AM251 (1 μm) and SR14171 (1 μm) consistently increased the frequency of sIPSCs in VP neurones without affecting their amplitude, suggesting a tonic CB presence. This effect on frequency was independent of action potential activity, and blocked by chelating intracellular calcium with 10 mm ethylene glycol tetraacetic acid (EGTA). AM251 also increased the amplitude of eIPSCs and decreased the paired-pulse ratio (PPR) in VP neurones-effects that were completely blocked with even low (1 mm EGTA) internal calcium chelation. Bouts of evoked firing of VP neurones consistently suppressed sIPSCs but had no effect on eIPSCs or the PPR. This depolarisation-induced suppression of IPSCs was reduced by AM251, and was totally blocked by 10 μm of the mixed vasopressin/oxytocin antagonist, Manning compound. We then tested the effect of vasopressin on IPSCs at the same time as blocking CB1 receptors. Vasopressin (10-100 nm) inhibited sIPSC frequency but had no effect on sIPSC or eIPSC amplitudes, or on the PPR, in the presence of AM251. Taken together, these results suggest a tonic, pre-synaptic inhibitory modulation of IPSCs in VP neurones by CBs that is largely dependent on post-synaptic calcium, and an inhibitory effect of VP on IPSCs that is independent of CB release.

GABA is excitatory in adult vasopressinergic neuroendocrine cells

Neuronal excitability in the adult brain is controlled by a balance between synaptic excitation and inhibition mediated by glutamate and GABA, respectively. While generally inhibitory in the adult brain, GABA(A) receptor activation is excitatory under certain conditions in which the GABA reversal potential is shifted positive due to intracellular Cl(-) accumulation, such as during early postnatal development and brain injury. However, the conditions under which GABA is excitatory are generally either transitory or pathological. Here, we reveal GABAergic synaptic inputs to be uniformly excitatory in vasopressin (VP)-secreting magnocellular neurons in the adult hypothalamus under normal conditions. The GABA reversal potential (E(GABA)) was positive to resting potential and spike threshold in VP neurons, but not in oxytocin (OT)-secreting neurons. The VP neurons lacked expression of the K(+)-Cl(-) cotransporter 2 (KCC2), the predominant Cl(-) exporter in the adult brain. The E(GABA) was unaffected by inhibition of KCC2 in VP neurons, but was shifted positive in OT neurons, which express KCC2. Alternatively, inhibition of the Na(+)-K(+)-Cl(-) cotransporter 1 (NKCC1), a Cl(-) importer expressed in most cell types mainly during postnatal development, caused a negative shift in E(GABA) in VP neurons, but had no effect on GABA currents in OT neurons. GABA(A) receptor blockade caused a decrease in the firing rate of VP neurons, but an increase in firing in OT neurons. Our findings demonstrate that GABA is excitatory in adult VP neurons, suggesting that the classical excitation/inhibition paradigm of synaptic glutamate and GABA control of neuronal excitability does not apply to VP neurons.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

Dehydration followed by sham rehydration contributes to reduced neuronal activation in vasopressinergic supraoptic neurons after water deprivation

This experiment tested the role of oropharyngeal and gastric afferents on hypothalamic activation in dehydrated rats instrumented with gastric fistulas and allowed to drink water or isotonic saline compared with euhydrated controls (CON). Rats were water-deprived for 48 h (48 WD) or 46 h WD with 2 h rehydration with water (46+W) or isotonic saline (46+S). 46+W and 46+S rats were given water with fistulas open (46+WO/46+SO, sham) or closed (46+WC/46+SC). Compared with CON, water deprivation increased and water rehydration decreased plasma osmolality, while sham rehydration had no effect. Water deprivation increased c-Fos staining in the lamina terminalis. However, none of the sham or rehydration treatments normalized c-Fos staining in the lamina terminalis. Analysis of AVP and c-Fos-positive neurons in the supraoptic nucleus (SON) revealed reduced colocalization in 46+WO and 46+SC rats compared with 48 WD and 46+SO rats. However, 46+WO and 46+SC rats had higher c-Fos staining in the SON than 46+WC or CON rats. Examination of c-Fos in the perinuclear zone (PNZ) revealed that sham and rehydrated rats had increased c-Fos staining to CON, while 48 WD and 46+SO rats had little or no c-Fos staining in this region. Thus, preabsorptive reflexes contribute to the regulation of AVP neurons in a manner independent of c-Fos expression in the lamina terminalis. Further, this reflex pathway may include inhibitory interneurons in the PNZ region surrounding the SON.

Brain activation of reading Korean words and recognizing pictures by Korean native speakers: A functional magnetic resonance imaging study

Even though the Korean words are characterized as phonemes like other alphabetic languages, their shape resembles much more morphemes like Chinese characters. The main purpose of the study is to explore neural mechanisms of reading Korean words and recognizing pictures by Korean native speakers using functional magnetic resonance imaging technique. In the experimental results, the authors could see commonly activated areas in occipito-temporal region bilaterally, whereas frontal and temporal region was activated only while reading Korean words. Left middle frontal activation of Korean words was regarded to be involved in the phonological and semantic processing. Right anterior cingulate (BA 32) activation seems to be related with language and sound organization and superior temporal (BA 29) activation might be involved in the processing of phonological system to which tonal information is attached. Right medial frontal (BA 8) activation was reported in the results. The authors suggest the activation of this area is related with nonverbal visual higher order control or visuospatial analysis of Korean words in their experimental tasks.

Serotonergic mechanisms of the lateral parabrachial nucleus in renal and hormonal responses to isotonic blood volume expansion

This study investigated the involvement of serotonergic mechanisms of the lateral parabrachial nucleus (LPBN) in the control of sodium (Na+) excretion, potassium (K+) excretion, and urinary volume in unanesthetized rats subjected to acute isotonic blood volume expansion (0.15 M NaCl, 2 ml/100 g of body wt over 1 min) or control rats. Plasma oxytocin (OT), vasopressin (VP), and atrial natriuretic peptide (ANP) levels were also determined in the same protocol. Male Wistar rats with stainless steel cannulas implanted bilaterally into the LPBN were used. In rats treated with vehicle in the LPBN, blood volume expansion increased urinary volume, Na+ and K+ excretion, and also plasma ANP and OT. Bilateral injections of serotonergic receptor antagonist methysergide (1 or 4 μg/200 ηl) into the LPBN reduced the effects of blood volume expansion on increased Na+ and K+ excretion and urinary volume, while LPBN injections of serotonergic 5-HT2a/HT2c receptor agonist, 2.5-dimetoxi-4-iodoamphetamine hydrobromide (DOI; 1 or 5 μg/200 ηl) enhanced the effects of blood volume expansion on Na+ and K+ excretion and urinary volume. Methysergide (4 μg) into the LPBN decreased the effects of blood volume expansion on plasma ANP and OT, while DOI (5 μg) increased them. The present results suggest the involvement of LPBN serotonergic mechanisms in the regulation of urinary sodium, potassium and water excretion, and hormonal responses to acute isotonic blood volume expansion.

Differences in spike train variability in rat vasopressin and oxytocin neurons and their relationship to synaptic activity

The firing pattern of magnocellular neurosecretory neurons is intimately related to hormone release, but the relative contribution of synaptic versus intrinsic factors to the temporal dispersion of spikes is unknown. In the present study, we examined the firing patterns of vasopressin (VP) and oxytocin (OT) supraoptic neurons in coronal slices from virgin female rats, with and without blockade of inhibitory and excitatory synaptic currents. Inhibitory postsynaptic currents (IPSCs) were twice as prevalent as their excitatory counterparts (EPSCs), and both were more prevalent in OT compared with VP neurons. Oxytocin neurons fired more slowly and irregularly than VP neurons near threshold. Blockade of Cl- currents (including tonic and synaptic currents) with picrotoxin reduced interspike interval (ISI) variability of continuously firing OT and VP neurons without altering input resistance or firing rate. Blockade of EPSCs did not affect firing pattern. Phasic bursting neurons (putative VP neurons) were inconsistently affected by broad synaptic blockade, suggesting that intrinsic factors may dominate the ISI distribution during this mode in the slice. Specific blockade of synaptic IPSCs with gabazine also reduced ISI variability, but only in OT neurons. In all cases, the effect of inhibitory blockade on firing pattern was independent of any consistent change in input resistance or firing rate. Since the great majority of IPSCs are randomly distributed, miniature events (mIPSCs) in the coronal slice, these findings imply that even mIPSCs can impart irregularity to the firing pattern of OT neurons in particular, and could be important in regulating spike patterning in vivo. For example, the increased firing variability that precedes bursting in OT neurons during lactation could be related to significant changes in synaptic activity.

Neuropeptide FF distribution in the human and rat forebrain: a comparative immunohistochemical study

Neuropeptide FF (NPFF) is an octapeptide implicated in a variety of physiological functions, including nociception, cardiovascular responses, and neuroendocrine regulation. The NPFF gene and its mRNA are highly conserved across species. A comparative study of NPFF distribution in the human and rat forebrain was carried out by using single NPFF and double NPFF + vasopressin (VP) immunohistochemistry. NPFF is extensively localized within neurochemical circuits of human and rat forebrain. Semiquantitative analysis revealed that the densities of NPFF cells and fibers in many forebrain nuclei in the human correlate well with those observed for the same structures in the rat. High numbers of NPFF positive neurons in the dorsomedial hypothalamic nucleus and a dense plexus of NPFF fibers surrounding the fornix within the bed nucleus of the stria terminalis were identified in the human and rat forebrain. Within the hypothalamus of both species, dense NPFF innervation was observed in the perinuclear zone of the supraoptic nucleus (SO) just dorsolateral to the VP-positive neurons. Extensive NPFF innervation of ventricular ependyma and brain microvasculature were common for both species. At the same time, obvious differences in NPFF localization between the two species were also apparent. For example, in contrast to the rat SO, no NPFF- or NPFF- + VP-immunostained cells were observed in the human SO. Knowledge of NPFF neuroanatomical localization in the human brain and the relationship of these observations to those in the rat brain may provide insight into the role of this peptide in central cardiovascular and neuroendocrine regulation.

Remodeling of astrocytes, a prerequisite for synapse turnover in the adult brain? Insights from the oxytocin system of the hypothalamus

Neurons, including their synapses, are generally ensheathed by fine processes of astrocytes, but this glial coverage can be altered under different physiological conditions that modify neuronal activity. Changes in synaptic connectivity accompany astrocytic transformations so that an increased number of synapses are associated with reduced astrocytic coverage of postsynaptic elements, whereas synaptic numbers are reduced on reestablishment of glial coverage. A system that exemplifies activity-dependent structural synaptic plasticity in the adult brain is the hypothalamo-neurohypophysial system, and in particular, its oxytocin component. Under strong, prolonged activation (parturition, lactation, chronic dehydration), extensive portions of somatic and dendritic surfaces of magnocellular oxytocin neurons are freed of intervening astrocytic processes and become directly juxtaposed. Concurrently, they are contacted by an increased number of inhibitory and excitatory synapses. Once stimulation is over, astrocytic processes again cover oxytocinergic surfaces and synaptic numbers return to baseline levels. Such observations indicate that glial ensheathment of neurons is of consequence to neuronal function, not only directly, for example by modifying synaptic transmission, but indirectly as well, by preparing neuronal surfaces for synapse turnover.

Neuropeptide FF (NPFF) control of magnocellular neurosecretory cells of the rat hypothalamic paraventricular nucleus (PVN)

Neuropeptide FF (NPFF) is an octapeptide belonging to an extended family of RF amide peptides that have been implicated in a wide variety of physiological functions in the brain. NPFF and its receptors are abundantly expressed in the rat brain and spinal cord including the hypothalamic paraventricular nucleus (PVN), an autonomic nucleus critical for the secretion of neurohormones and the regulation of sympathetic outflow. In this study, we sought to examine the effects of NPFF on GABAergic inhibitory synaptic input to magnocellular neurosecretory cells (MNCs) of the PVN, which secrete the neurohormones, vasopressin and oxytocin from their terminals in the neurohypophysis. Whole cell patch clamp recordings under voltage clamp conditions were performed from PVN MNCs in the brain slice. Bicuculline-sensitive inhibitory postsynaptic currents (IPSCs) were isolated in the presence of glutamate receptor blockers. In tetrodotoxin, NPFF (5 microM) caused an increase in frequency, but not amplitude of miniature inhibitory postsynaptic currents (mIPSCs) in MNCs indicating a presynaptic locus of action for this peptide. Intracerebroventricular application of NPFF resulted in an activation of GABAergic neurons located adjacent to the PVN as revealed by immunohistochemistry for Fos protein and in situ hybridization for glutamic acid decarboxylase (GAD67) mRNA. Based on these observations we conclude that NPFF facilitates inhibitory input to MNCs of the PVN via GABAergic interneurons located in immediate vicinity of the nucleus. These findings provide a cellular and anatomic basis for the NPFF-induced inhibition of vasopressin release has been reported consequent to hypovolemia and hyperosmolar stimulation.

GABA and glutamate receptors in the horizontal limb of diagonal band of Broca (hDB): effects on cardiovascular regulation

The horizontal limb of diagonal band of Broca (hDB) is a part of the limbic system. It has been shown that microinjection of L-glutamate into the hDB elicited cardiovascular depressive responses in anesthetized rats and pressor effect in unanesthetized rats. But the role of glutamate receptor subtypes has not yet been investigated. In addition the role of the GABAergic system of the hDB in cardiovascular responses is not known. Therefore, we examined the cardiovascular responses elicited by glutamate and GABA receptors in the hDB by using their agonists and antagonists. Drugs (50 nl) were microinjected into the hDB of anaesthetized rats. Blood pressure and heart rate were recorded before and throughout each experiment. The average changes in the mean arterial pressure and heart rate at different intervals were compared both within each case group and between the case and control groups using repeated measures of ANOVA. Microinjection of GABA(A) receptor antagonist, bicuculline methiodide (BMI, 1 mM) increased both the mean arterial pressure and heart rate, and muscimole, a GABA(A) agonist (500 pmol) caused a significant decrease in the mean arterial pressure and heart rate. Microinjection of L-glutamate (0.25 M) into the hDB resulted in a maximum decrease of the mean arterial pressure of 24.4 +/- 3.7 mmHg and heart rate of 25.2 +/- 3.08 beats/min. Injection of AP5, an antagonist of glutamate NMDA receptor (1 and 2.5 mM), and CNQX, an antagonist of glutamate AMPA receptor (0.5 and 1 mM) caused small, nonsignificant changes of the heart rate and the blood pressure. Either AP5 or CNQX when coinjected with glutamate abolished the depressor effect of glutamate, suggesting that simultaneous activation of both glutamate receptors is necessary for the effect of glutamate to emerge. The depressor effect of the glutaminergic system of the hDB on the cardiovascular system was similar to the previous studies. For the first time, the effects of CNQX, AP5, BMI, and muscimole have been shown in this study.

Neurochemical brain groups activated after an isotonic blood volume expansion in rats

In order to establish the involvement of particular neurochemical brain groups in the response to blood volume expansion, we analyzed Fos-labeling in combination with immunolabeling for serotonin, tyrosine hydroxylase, vasopressin and oxytocin, 90 min after a sham or i.v. isotonic blood volume expansion (BVE) in unanesthetized, unrestrained rats. We also examined the changes in concentration of oxytocin, atrial natriuretic peptide and vasopressin plasma, induced by blood volume load, to confirm our previous studies. The results demonstrate the participation of specific paraventricular and supraoptic nucleus groups of cells (oxytocinergic-vasopressinergic), serotoninergic dorsal raphe nucleus cells and catecholaminergic A1/A2/A6 groups (in the caudal ventrolateral medulla, nucleus of the solitary tract and locus coeruleus respectively), in the regulatory response to BVE. They provide detailed neuroanatomical evidence to support previous observations showing the contribution of these neurochemical systems in the neural, behavioral and endocrine response to isotonic BVE.

Distribution of the neuropeptide FF1 receptor (hFF1) in the human hypothalamus and surrounding basal forebrain structures: immunohistochemical study

Neuropeptides with C-terminal RFamide and their receptors NPFF1 (FF1) and NPFF2 (FF2) have been implicated in a wide variety of functions, including nociception and autonomic and neuroendocrine regulation. Recent studies indicate that the FF1, but not FF2, mRNA is highly expressed in the human hypothalamus. In the present study, localization of FF1 in the human hypothalamus and surrounding regions was studied immunohistochemically by using an antibody against human FF1 (hFF1). Brain sections from healthy 30-50-year-old individuals were used for hFF1 immunohistochemistry. The highest density of hFF1-stained cells was found in the posterior division of the bed nucleus of the stria terminalis and in the zona incerta. A moderate density of cells was observed in the perifornical nucleus, infundibular nucleus, tuberomammillary nucleus, and lateral tuberal nucleus. A lesser density was revealed in the dorsomedial hypothalamic nucleus, basal nucleus of Meynert, and anterior amygdaloid area. Only scattered hFF1 cells were found in the suprachiasmatic nucleus and hypothalamic paraventricular nucleus. hFF1 cells and fibers were absent in the supraoptic and mammillary nuclei. Single and double strands of hFF1-immunopositive punctate varicosities marked cellular processes of different caliber. The density of hFF1-immunostained fiber networks did not always coincide with that of hFF1-immunostained cells. hFF1 immunoreactivity was also found in the wall of blood vessels within most brain areas studied. Localization of hFF1 in discrete regions of the hypothalamus and extended amygdala may provide important insights into the role of amidated neuropeptides in central autonomic and neuroendocrine control in the human brain.

Vasopressin response to osmotic and hemodynamic stress: neurotransmitter involvement

Osmotic and hemodynamic stress are the two primary regulators of vasopressin (VP) release from the posterior pituitary. The pathways providing information about plasma osmolality and blood pressure or blood volume are distinct and utilize different chemical neurotransmitters. Osmotic regulation of VP release is dependent upon afferents from the lamina terminalis region. Glutamate is an important transmitter in this system and angiotensinergic afferents from this region to the VP neurons modulate responses to osmotic challenges. Hemodynamic information is transmitted to the VP neurons via multisynaptic pathways from the brainstem with the A1 catecholamine neurons of the ventrolateral medulla providing the final link for information about decreases in blood pressure and volume. Several neurotransmitters and neuropeptides are expressed in the A1 neurons including norepinephrine (NE), ATP, neuropeptide Y, and substance P. The impact of co-release of these agents on VP release is reviewed and the potential physiological significance is discussed.

Autonomic and neuroendocrine actions of adrenomedullin in the brain: mechanisms for homeostasis

In addition to its role as a potent vasodilator, adrenomedullin (ADM) affects an animal's physiological status through its effects in the brain. We have shown that circulating ADM activates neurons, including nitric oxide (NO)-producing neurons, in autonomic centers of the brain such as the hypothalamic paraventricular nucleus (PVN). Systemic ADM gains access to the brain through the area postrema (AP), a brainstem circumventricular organ, and the PVN is a major target of these ADM-sensitive AP neurons. Neurons expressing the preproADM (ppADM) gene are distributed throughout the brain, with high levels in autonomic centers. Lipopolysaccharide (LPS, immune stress), restraint (psychological stress), and 24 h dehydration all down-regulate ppADM gene expression in different subsets of autonomic centers. Receptor-activity-modifying protein (RAMP) 2 and RAMP3, ADM receptor subunits, are expressed in autonomic centers including the PVN and supraoptic nucleus. Intracerebroventricular injections of ADM increase arterial pressure, heart rate, tyrosine hydroxylase mRNA levels in the locus coeruleus, plasma levels of ACTH, and NO production in the hypothalamus. ADM excites putative GABAergic and cholinergic neurons in dissociated cells from a basal forebrain integrative center, the diagonal band of Broca. These results demonstrate that the signalling components necessary for ADM to influence physiological systems are present in the brain and that ADM is an important transmitter of brain autonomic pathways which are involved in regulating homeostatic balance.

Direct catecholaminergic-cholinergic interactions in the basal forebrain. III. Adrenergic innervation of choline acetyltransferase-containing neurons in the rat

The central adrenergic neurons have been suggested to play a role in the regulation of arousal and in the neuronal control of the cardiovascular system. To provide morphological evidence that these functions could be mediated via the basal forebrain, we performed correlated light and electron microscopic double-immunolabeling experiments using antibodies against phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase, the synthesizing enzymes for adrenaline and acetylcholine, respectively. Most adrenergic/cholinergic appositions were located in the horizontal limb of diagonal band of Broca, within the substantia innominata, and in a narrow band bordering the substantia innominata and the globus pallidus. Quantitative analysis indicated that cholinergic neurons of the substantia innominata receive significantly higher numbers of adrenergic appositions than cholinergic cells in the rest of the basal forebrain. In the majority of cases, the ultrastructural analysis revealed axodendritic asymmetric synapses. By comparing the number and distribution of dopamine beta-hydroxylase (DBH)/cholinergic appositions, described earlier, with those of PNMT/cholinergic interactions in the basal forebrain, it can be concluded that a significant proportion of putative DBH/cholinergic contacts may represent adrenergic input. Our results support the hypothesis that the adrenergic/cholinergic link in the basal forebrain may represent a critical component of a central network coordinating autonomic regulation with cortical activation.

Intrapericardial procaine affects volume expansion-induced fos immunoreactivity in unanesthetized rats

Acute volume expansion is associated with a specific pattern of Fos expression and the goal of the present study was to evaluate the contribution of cardiac receptors to this response. Adult male rats were instrumented with pericardial catheters introduced at the level of the thymus. Rats were also catheterized for measuring blood pressure, heart rate, central venous pressure, and intravenous infusion. Each rat received a 200-microl intrapericardial (i.p.c) injection of 2% procaine or 0.9% NaCl. Rats were then volume expanded with isotonic saline (10% body weight in 10 min) or given a control infusion (0.01 ml/min for 10 min). Ninety minutes after the start of the infusion, the rats were anesthetized and perfused transcardially. Their brains were sectioned and processed for Fos, dopamine-beta-hydroxylase, and oxytocin immunocytochemistry. Volume expansion plus i.p.c. saline produced a significant increase in Fos expression in the nucleus of the solitary tract, the ventrolateral medulla, the area postrema, the locus coeruleus, the paraventricular nucleus of the hypothalamus, the perinuclear zone of the supraoptic nucleus, and oxytocin neurons in the supraoptic nucleus. The i.p.c. procaine significantly blocked Fos expression produced by the volume expansion in the all of the regions examined except for the area postrema and the SON oxytocin neurons.

Fos immunoreactivity in the diagonal band and the perinuclear zone of the supraoptic nucleus after hypertension and hypervolaemia in unanaesthetized rats

We used Fos immunocytochemistry to study the effects of hypertension and hypervolaemia on neurones in the diagonal band of Broca and the perinuclear zone of the supraoptic nucleus, two nuclei that are both involved in the baroreceptor regulation of vasopressin neurones in the supraoptic nucleus. In addition, we used sino-aortic denervation to examine the role of arterial baroreceptors in the response to these haemodynamic changes. Sham-operated and sino-aortic denervated rats were infused with phenylephrine sufficient to increase blood pressure for 2 h. Control rats were infused with the same volume of isontonic saline. Only Sham sino-aortic denervated rats showed reflex bradycardia in response to the increased blood pressure. Volume expansion was produced by infusing the rats with isotonic saline equal to 10% of their body weight for 10 min, which significantly increased central venous pressure. In the diagonal band of Broca and the perinuclear zone, the number of Fos-positive neurones was significantly increased after phenylephrine infusion. Sino-aortic denervation blocked the significant increase in both regions. After volume expansion, a significant increase in Fos staining was observed only in the perinuclear zone of the supraoptic nucleus. This increase was not blocked by sino-aortic denervation. Our results indicate that both the diagonal band of Broca and the perinuclear zone of the supraoptic nucleus are activated by stimulating arterial baroreceptors; however, the perinuclear zone of the supraoptic nucleus is stimulated during volume expansion. Furthermore, the activation of perinuclear zone of the supraoptic nucleus after volume expansion is not dependent on intact arterial baroreceptors.

Oxytocin-secreting neurons: A physiological model of morphological neuronal and glial plasticity in the adult hypothalamus

Oxytocin-secreting neurons of the hypothalamoneurohypophysial system undergo reversible morphological changes whenever they are strongly stimulated. In the hypothalamus, such structural plasticity is represented by modifications in the size and shape of their somata and dendrites, in the extent to which their surfaces are covered by glia, and in the density of their synapses. In the neurohypophysis, there is a parallel reduction in glial (pituicyte) coverage of their axons together, with retraction of pituicyte processes from the perivascular basal lamina and an increase in the number and size of their terminals. These changes occur rapidly, within a few hours. On the other hand, the system returns to its prestimulated condition on arrest of stimulation at a rate that depends on the length of time it has remained activated. Such neuronal-glial changes have several functional consequences. In the hypothalamic nuclei, reduction in astrocytic coverage of oxytocinergic neurons and their synapses modifies extracellular ionic homeostasis and glutamate clearance and, therefore, their overall excitability. Since it results in extensive dendritic bundling, it may also lead to ephaptic interactions and may facilitate dendritic electrotonic coupling. A most important indirect effect may be to permit synaptic remodeling that occurs concomitantly and that results in significant increases in the number of excitatory and inhibitory synapses driving their activity. In the stimulated neurohypophysis, glial retraction results in increased levels of extracellular K+ which can enhance neurohormone release while an enlarged neurovascular contact zone may facilitate diffusion of neurohormone into the circulation. Ongoing work aims to unravel the cell mechanisms and factors underlying such plasticity and has revealed that neurons and glia of the hypothalamoneurohypophysial system continue to express juvenile molecular features associated with similar neuronglial interactions and synaptic events during development and regeneration. They include strong expression of cell surface adhesion molecules like F3/contactin and polysialylated neural cell adhesion molecule, extracellular matrix glycoproteins like tenascin C, and cytoskeletal proteins like vimentin and microtubule-associated protein 1D. Some of these molecules reach the cell surface constitutively while others follow the activity-dependent regulated pathway. We consider many of these molecular features permissive, allowing oxytocin neurons and their glia to undergo morphological remodeling throughout life, provided the proper stimulus intervenes. In the hypothalamic nuclei, one such stimulus is centrally released oxytocin; in the neurohypophysis, an adrenergic, cAMP-mediated mechanism appears responsible.

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.

Taurine selectively modulates the secretory activity of vasopressin neurons in conscious rats

Previous experiments have shown that a 10-min forced swimming session triggers the release of vasopressin from somata and dendrites, but not axon terminals, of neurons of the hypothalamic-neurohypophysial system. To further investigate regulatory mechanisms underlying this dissociated release, we forced male Wistar rats to swim in warm (20 degrees C) water and monitored release of the potentially inhibitory amino acids gamma amino butyric acid (GABA) and taurine into the hypothalamic supraoptic nucleus using microdialysis. Forced swimming caused a significant increase in the release of taurine (up to 350%; P < 0.05 vs. prestress release), but not GABA. To reveal the physiological significance of centrally released taurine, the specific taurine antagonist 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide was administered into the supraoptic nucleus via retrodialysis. Administration of this antagonist caused a significant increase in the release of vasopressin within the supraoptic nucleus and into the blood both under basal conditions and during stress (up to 800%; P < 0.05 vs. basal values), without affecting hypothalamic or plasma oxytocin. Local administration of the GABA(A) receptor antagonist bicuculline, in contrast, failed to influence vasopressin secretion at either time point. In a separate series of in vivo electrophysiological experiments, administration of the same dosage of the taurine antagonist into the supraoptic nucleus via microdialysis resulted in an increased electrical activity of identified vasopressinergic, but not oxytocinergic, neurons. Taken together our data demonstrate that taurine is released within the supraoptic nucleus during physical/emotional stress. Furthermore, at the level of the supraoptic nucleus, taurine inhibits not only the electrical activity of vasopressin neurons but also acts as an inhibitor of both central and peripheral vasopressin secretion during different physiological states.

Cardiovascular regulation of vasopressin neurons in the supraoptic nucleus

This paper reviews the regulation of hypothalamic vasopressin and oxytocin neurosecretory cells in the neural response to plasma volume expansion. Many questions remain unanswered regarding how an increase in volume affects neurohypophysial hormone secretion, what receptors are important in mediating this response, and which neural pathways are responsible for conveying the signal from those receptors to the hypothalamus. Plasma volume expansion activates regions of the central nervous system associated with inhibition of vasopressin release, oxytocin secretion, and inhibition of sympathetic nerve activity. Cardiac receptors, not arterial baroreceptors, are primarily responsible for activation of the regions associated with regulation of vasopressin secretion and sympathetic outflow. Other stimuli that as yet are undefined account for activation of oxytocin-secreting neurons. Electrophysiology experiments have measured the inhibition of vasopressin-secreting magnocellular neurons in the supraoptic nucleus by select stimulation of cardiac receptors in the caval-atrial junction. Further experiments suggest that the perinuclear zone, a population of neurons surrounding the supraoptic nucleus, is a necessary part of the pathway by which caval-atrial stretch decreases the excitability of vasopressin neurons. The perinuclear zone is also a necessary synapse for arterial baroreceptor-mediated inhibition of vasopressin neurons. This suggests that the neural pathways that inhibit vasopressin release in response to an increase in blood pressure and an increase in blood volume may overlap at the perinuclear zone of the supraoptic nucleus. Finally, the integration of various neural pathways activated by multiple receptors to ultimately determine the activity of magnocellular neurons and vasopressin secretion is discussed.

Upregulation of iNOS expression and phosphorylation of eIF-2alpha are paralleled by suppression of protein synthesis in rat hypothalamus in a closed head trauma model

When the inducible form of nitric oxide synthase (iNOS) is expressed after challenge to the nervous system, it results in abnormally high concentrations of nitric oxide (NO). Under such conditions, NO could phosphorylate the eukaryotic translation initiation factor (eIF)-2alpha, thus suppressing protein synthesis in neurons that play a role in endocrine and autonomic functions. Using the Marmarou model of traumatic brain injury (TBI), we observed a rapid increase (at 4 h after TBI) of iNOS mRNA in magno- and parvocellular supraoptic and paraventricular neurons, declining gradually by approximately 30% at 24 h and by approximately 80% at 48 h. Western analysis indicated a trend towards increased iNOS protein synthesis at 4 h, which peaked at 8 h, and tended to decrease at the later time points. At the same time points, we detected immunocytochemically the phosphorylated form of eIF-2alpha (eIF-2alpha[P]) as cytoplasmic and more often as nuclear labeling. The incidence of double-labeled [iNOS and eIF-2alpha(P)] neuronal profiles, particularly at 24 h and 48 h after TBI, was high. De novo protein synthesis assessed quantitatively after infusion of 35S methionine/cysteine was reduced by approximately 20% at 4 h, remained depressed at 24 h, and did not return to control levels up to 48 h following the trauma. The results suggest that iNOS may trigger phosphorylation of eIF-2alpha, which in turn interferes with protein synthesis at the translational (ribosomal complex) and transcriptional (chromatin) levels. The depression in protein synthesis may include downregulation of iNOS itself, which could be an autoregulatory inhibitory feedback mechanism for NO synthesis. Excessive amounts of NO may also participate in dysfunction of hypothalamic circuits that underlie endocrine and autonomic alterations following TBI.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Lesion of the perinuclear zone attenuates cardiac sensitivity of vasopressinergic supraoptic neurons

Discrete stretch of the caval-atrial junction decreases the activity of vasopressin-secreting neurons in the supraoptic nucleus (SON). The perinuclear zone (PNZ) of the SON is necessary for inhibition of vasopressin neurons following an increase in blood pressure. To determine whether the PNZ is necessary for cardiopulmonary regulation of vasopressin neurons, male rats received three unilateral injections of the excitotoxin ibotenic acid (n = 9) or phosphate-buffered saline vehicle (n = 10) into the PNZ. Extracellular activity of antidromically identified phasic vasopressin neurons in the ipsilateral SON was recorded. Of the 26 neurons recorded from vehicle-injected rats 26 were inhibited by an increase in blood pressure and 22 of those neurons were sensitive to caval-atrial distension. Of the neurons recorded from PNZ-lesion rats, only 12 of 29 were inhibited by an increase in blood pressure (P < 0.05), and only 11 neurons were sensitive to caval-atrial stretch (P < 0.05). Functional lesion of the PNZ significantly attenuates both arterial and cardiopulmonary baroreceptor-mediated inhibition of supraoptic vasopressin neurons, suggesting that the PNZ is a necessary component of both pathways.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

Baroreceptor sensitivity of rat supraoptic vasopressin neurons involves noncholinergic neurons in the DBB

Previous studies suggest that cholinergic neurons in the diagonal band of Broca (DBB) participate in the baroreceptor-mediated inhibition of phasic vasopressin neurons in the supraoptic nucleus (SON). To test this hypothesis, extracellular recordings were obtained from putative vasopressin SON neurons of anesthetized rats injected with the cholinergic immunotoxin 192 IgG-saporin (0.8 microg/microl) in the DBB. Baroreceptor sensitivity of neurons was tested with brief phenylephrine-induced (10 microg/10 microl iv) increases in blood pressure of at least 40 mmHg. In rats injected with vehicle or unconjugated saporin, 19 of 21 and 18 of 20 phasic neurons, respectively, were inhibited by increased blood pressure. In rats injected with 192 IgG-saporin, which significantly reduced the number of choline acetyltransferase (ChAT)-positive DBB neurons, 33 of 36 phasic neurons were inhibited. Normal rats and rats with DBB saporin injections received rhodamine bead injections into the perinuclear zone (PNZ) to retrogradely label DBB neurons, and their brains were stained for ChAT. ChAT-positive DBB neurons were not retrogradely labeled from the PNZ. Together, these results indicate that the pathway relaying baroreceptor information to the SON involves noncholinergic DBB neurons.

Role of the locus ceruleus in baroreceptor regulation of supraoptic vasopressin neurons in the rat

The goal of this study was to identify the source of baroreceptor-related noradrenergic innervation of the diagonal band of Broca (DBB). Male Sprague-Dawley rats underwent sinoaortic denervation (SAD, n = 13) or sham SAD surgery (n = 13). We examined Fos expression produced by baroreceptor activation and dopamine-beta-hydroxylase immunofluorescence in hindbrain regions that contain noradrenergic neurons. Baroreceptors were stimulated by increasing blood pressure >40 mmHg with phenylephrine (10 microgram. kg(-1). min(-1) iv) in sham SAD and SAD rats. Controls were infused with 0.9% saline. Only the locus ceruleus (LC) demonstrated a baroreceptor-dependent increase in Fos immunoreactivity in dopamine-beta-hydroxylase-positive neurons. In a second experiment, normal rats received rhodamine-labeled microsphere injections in the DBB (n = 12) before phenylephrine or vehicle infusion. In these experiments, only the LC consistently contained Fos-positive cells after phenylephrine infusion that were retrogradely labeled from the DBB. Finally, we lesioned the LC with ibotenic acid and obtained extracellular recordings from identified vasopressin neurons in the supraoptic nucleus. LC lesions significantly reduced the number of vasopressin neurons that were inhibited by acute baroreceptor stimulation. Together, these results suggest that noradrenergic neurons in the LC participate in the baroreflex activation of the DBB and may thus be important in the baroreflex inhibition of vasopressin-releasing neurons in the supraoptic nucleus.

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.

Differential increase in Fos immunoreactivity in hypothalamic and septal nuclei by arginine8-vasopressin and desglycinamide9-arginine8-vasopressin

Subcutaneous or intracerebroventricular injection of either arginine8-vasopressin or desglycinamide9-arginine8-vasopressin has been shown to facilitate memory, reduce or reverse the effects of amnesic drugs, and maintain tolerance to some effects of ethanol. These actions of vasopressin (and, by inference, of desglycinamide9-arginine8-vasopressin) are mediated by vasopressin V1 receptors in brain, via a c-fos-dependent mechanism, but the receptors at which the desglycinamide analog acts have not been identified. The precise central sites are also not known, but evidence of several types suggested the anterior hypothalamus and septum as probable loci of vasopressin action. In the present work, this question was studied by immunocytochemistry, using antibodies against Fos and Fos-like proteins. The numbers of Fos-immunoreactive nuclei were counted in several related brain regions and structures, after administration of arginine8-vasopressin, des-Gly9-[Arg8]-vasopressin or saline. A subcutaneous injection of vasopressin, but not of saline, enhanced Fos expression in the paraventricular, supraoptic and suprachiasmatic nuclei of the hypothalamus, but the desglycinamide analog stimulated Fos expression only in the suprachiasmatic nucleus. Vasopressin injection significantly increased the number of Fos-immunoreactive cells in the intermediate lateral septum, medial septum, and dorsal and ventral divisions of the lateral septum. In contrast, the desglycinamide analog increased the numbers of Fos-immunoreactive cells in the dorsal and intermediate portions of the lateral septum, but caused no change in the medial septum, and a decrease in the ventral portion of the lateral septum. Increased Fos expression was also found in the subfornical organ after subcutaneous injection of either vasopressin or the desglycinamide analog. Double labeling with antibodies against Fos protein and against vasopressin revealed that most of the vasopressin-induced Fos-immunoreactive cells in the supraoptic, paraventricular and suprachiasmatic hypothalamic nuclei are also vasopressin immunoreactive, i.e. they are vasopressin-producing neurons. These findings suggest that a circuit involving V1 receptors in the subfornical organ, connecting fibres to the suprachiasmatic nucleus, and vasopressinergic projections from the suprachiasmatic nucleus to the lateral septum, may play a central role in mediating the actions of both vasopressin and its desglycinamide analog in the maintenance of ethanol tolerance.

Projections of medullary and pontine noradrenergic neurons to the horizontal limb of the nucleus of diagonal band in the rat

Recent investigations in the rat have implicated a noradrenergic innervation to the horizontal nucleus of the diagonal band of Broca as a critical link in a neural circuit that conveys baroreceptor information centrally to inhibit the firing of vasopressin-secreting neurons in the hypothalamic supraoptic nucleus. In this study we used small intra-diagonal band injections of a retrograde tracer, rhodamine latex microspheres, in combination with tyrosine hydroxylase histochemistry to identify brainstem noradrenergic cells contributing to this innervation. In three cases where tracer injections were limited to the horizontal limb of the diagonal band, we observed 20-50 double-labelled neurons ipsilaterally in the dorsal part of the locus coeruleus (A6) and the caudal nucleus tractus solitarius (A2), and bilaterally in the caudal ventrolateral medulla (A1). Double-labelled neurons were also noted in the ventral tegmental area (dopaminergic A10 cell group). Although all major brainstem noradrenergic cell groups contribute fibers to the horizontal limb of the nucleus of diagonal band, data from physiological studies suggest that the noradrenergic A2 neurons in the nucleus tractus solitarius are the most likely pathway through which it receives this baroreceptor information.

Fos expression following isotonic volume expansion of the unanesthetized male rat

Cardiopulmonary afferents, baroreceptor afferents, or atrial natriuretic peptide binding to circumventricular organs may mediate the central response to volume expansion, a condition common to pregnancy, exercise training, and congestive heart failure. This study used Fos immunocytochemistry to examine brain regions activated by volume expansion. Male Sprague-Dawley rats were infused with isotonic saline equal to 10% of their body weight in 10 min followed by a maintenance infusion of 0.5 ml/min for 110 min. Control animals received 2-h infusions at 0.01 ml/min. Five minutes after the start of volume expansion, central venous pressure of expanded animals was significantly greater than control animals. The volume-expanded group exhibited significantly greater Fos activation (P < 0.05) in the area postrema, nucleus of the solitary tract, caudal ventrolateral medulla, paraventricular nucleus, supraoptic nucleus, and perinuclear zone of the supraoptic nucleus. Double labeling indicates that oxytocinergic neurons in the supraoptic nucleus are activated. Neurons in brain regions known to inhibit both sympathetic activity and vasopressin release show increased Fos expression following isotonic volume expansion.

Activation of presynaptic GABAB receptors inhibits evoked IPSCs in rat magnocellular neurons in vitro

1508-1517, 1998. Whole cell recordings (nystatin-perforated patch) were carried out on magnocellular neurons of the rat supraoptic nucleus (SON) to study the modulation of inhibitory postsynaptic currents (IPSCs) by gamma-aminobutyric acid-B (GABAB) receptors. Field stimulation adjacent to the SON in the presence of kynurenic acid, evoked monosynaptic GABAergic IPSCs. Baclofen reversibly reduced the amplitude of the IPSCs in a dose-dependent manner (EC50: 0.68 microM) without apparent effect on the holding current (Vh = -80 mV) or input resistance and altered neither the kinetic properties, nor the reversal potential of IPSCs. Concomittant to IPSC depression, baclofen enhanced the paired-pulse ratio for two consecutive IPSCs [interstimulus interval (ISI): 50 ms], an effect consistent with a presynaptic locus of action. Both actions of baclofen were abolished by CGP35348 (500 microM), a GABAB receptor antagonist. In testing for involvement of synaptically activated presynaptic GABAB receptors, we only recorded paired-pulse facilitation at most ISIs tested (50-500 ms), suggesting that the classical GABAB autoreceptors may not normally be activated in our conditions. However, enhancement of local GABA concentration by perfusion of a GABA uptake inhibitor (NO-711) revealed an action of endogenous GABA at these presynaptic GABAB receptors. The nonselective K+ channel blocker Ba2+ abolished baclofen's effect and pertussis toxin (PTX) pretreatment (200-500 ng/ml for 18-24 h) was ineffective in blocking the baclofen-induced inhibition, making an involvement of PTX-sensitive G protein unlikely. The present results show that presynaptic GABAB receptors that are coupled to PTX-insensitive G-proteins may be activated by endogenous GABA under conditions of reduced GABA uptake, thus regulating the inhibitory synaptic input to SON.

Glutamate microstimulation of local inhibitory circuits in the supraoptic nucleus from rat hypothalamus slices

The hypothesis of a local inhibitory input to the hypothalamic supraoptic nucleus was tested with combined glutamate microstimulation and whole cell patch-clamp recordings in slices from rat hypothalamus. Synaptic activity in supraoptic magnocellular neuroendocrine cells (MNCs) was monitored and glutamate microdrops were applied in the perinuclear region of the supraoptic nucleus to evoke firing of action potentials in putative presynaptic inhibitory cells. The effect of glutamate microdrops applied in the perinuclear region was tested on 57 supraoptic MNCs. In control conditions, spontaneous excitatory (EPSCs) and inhibitory (IPSCs) postsynaptic currents were observed at resting membrane potential in all MNCs tested. Glutamate microstimulation evoked an abrupt increase in the frequency and size of spontaneous IPSCs in eight MNCs. Forty-nine MNCs did not show any change in the inhibitory synaptic input. Microapplication of glutamate in the periphery of the supraoptic nucleus did not modify the amplitude or the frequency of spontaneous EPSCs in any of the 57 MNCs tested. In the group of eight MNCs that responded to glutamate microstimulation by an increase in inhibitory input, two types of responses were observed. Four MNCs showed an increase in both size and frequency of spontaneous IPSCs through the entire range of amplitude. In the other four MNCs, local glutamate stimulation produced a dramatic increase in the size of IPSCs and a lesser increase in the frequency of the smaller IPSCs. The potential effect of the glutamate-evoked increase in inhibitory input on the firing activity of MNCs was tested in current-clamp conditions. Intracellular current injection was applied to evoke firing of action potentials in six MNCs that had responded to local glutamate microstimulation by an increase in inhibitory input. Glutamate microdrop applications inhibited the evoked action potential firing in all six cells. These results suggest 1) that local inhibitory interneurons are present in the periphery of the supraoptic nucleus, 2) that they contain functional glutamate receptors, 3) that they form inhibitory synapses with supraoptic MNCs, and 4) that activation of these interneurons inhibits firing in MNCs. These results support the hypothesis that local inhibitory interneurons play a important role in the firing activity of supraoptic MNCs.

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus. J. Neurophysiol. 78: 2427-2437, 1997. Neurons in the perinuclear zone (PZ) of the supraoptic nucleus (SON) are thought to serve as interneurons and may mediate changes in neurohypophysial hormone release in response to physiological changes in blood pressure. However, the morphology and electrophysiological characteristics of PZ neurons are unknown. In the present study, PZ neurons from male and female rats were recorded intracellularly to determine some membrane properties, then filled with biocytin or biotinamide for morphological analysis. In general, PZ neurons had faster spikes than magnocellular SON neurons, and the great majority were characterized by a subthreshold depolarizing hump when depolarized from a hyperpolarized (less than -80 mV) membrane potential. In most neurons, this hump was similar to low-threshold spikes described in other CNS regions. Near-threshold, fast action potentials were clustered near the onset of these depolarizations. Conspicuously absent in all PZ neurons was the strong transient and subthreshold outward rectification characteristic of vasopressin and oxytocin neurons of the SON. These results suggest that PZ neurons are electrophysiologically distinct from neurosecretory neurons of the SON. No differences were found between male and female rats in any of the basic properties examined, including input resistance, membrane time constant, spike height, spike width, spike threshold, and the size of the spike afterhyperpolarization. Morphologically, PZ neurons were diverse but were divided into spiny and aspiny groups. Three spiny neurons and one aspiny neuron contributed an axonal projection to the SON characterized by varicosities suggestive of terminals. In the case of the three spiny neurons, the SON projection was clearly a minor collateral projection. The axon arborized in the PZ, but one or more branches were cut at the edge of the explant, indicating a longer projection. In the remaining neurons, no axonal projection to the SON was detected and several had axons leaving the explant. Some portion of the dendritic tree penetrated the SON in several neurons. The morphology of PZ neurons was thus heterogeneous and suggests that, for some cells at least, the projection to the SON may be a minor collateral component of a much wider axonal projection.

Neurones in the supraoptic nucleus of the rat are regulated by a projection from the suprachiasmatic nucleus

1. In the rat, projections from the suprachiasmatic nucleus (SCN) to the supraoptic nucleus (SON) of the hypothalamus were characterized in vivo using extracellular recordings and in slice preparations using both extracellular and whole-cell patch clamp recording. 2. Of 117 magnocellular neurones recorded in the SON in vivo, fifteen (13%) displayed a short latency excitation, sixty-eight (58%) a short latency inhibition, six (5%) were unresponsive and twenty-eight (24%) gave long latency responses following SCN stimulation. 3. The responses of putative vasopressin cells in the SON to SCN stimulation in vivo (4 out of 61 cells, 7% excited; 49 out of 61 cells, 80% inhibited) were significantly different from those of putative oxytocin cells (10 out of 50 cells, 20% excited and 16 out of 50 cells, 32% inhibited; P < 0.02, test for differences between proportions). 4. Recordings in vitro using patch technology in whole-cell mode showed both inward and outward currents in SON cells at holding potentials near resting membrane potential following stimulation of the SCN region. The outward currents could be blocked by bicuculline (10 microM; n = 7) and the inward currents were blocked by the non-NMDA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (5 microM; n = 4). 5. We conclude that there is a strong projection from the SCN to the SON with both inhibitory (GABAergic) and excitatory (glutamatergic) components which may regulate the daily changes in neurohypophysial hormone secretion.

Identification of estrogen receptor-containing neurons projecting to the rat supraoptic nucleus

Circulating estrogens influence the electrical and biosynthetic activity of the hypothalamic magnocellular neurons which synthesize vasopressin or oxytocin and regulate body fluid homeostasis and reproduction. As none of these magnocellular neurons express nuclear estrogen receptor in the rat, the present study has combined estrogen receptor immunocytochemistry with retrograde tracing techniques to examine whether the first-order neurons projecting to magnocellular neurons in the supraoptic nucleus may be receptive to estrogen. Green fluorescent latex microspheres (50 nl) were injected into the supraoptic nucleus of five ovariectomized rats. The largest numbers of retrogradely-labelled cells expressing estrogen receptor immunoreactivity were detected in the organum vasculosum of the lamina terminalis, anteroventral periventricular nucleus and medial preoptic nucleus where approximately 15% of all retrogradely-labelled cells were estrogen receptor-immunoreactive. Other prominent sites where double-labelled cells were detected were the median preoptic nucleus, subfornical organ, ventrolateral division of the hypothalamic ventromedial nucleus and the brainstem nucleus tractus solitarii. Triple labelling experiments in the caudal medulla revealed that the estrogen-receptive neurons of the nucleus tractus solitarii and ventrolateral medulla projecting to the supraoptic nucleus were not noradrenergic. These findings show that sub-populations of neurons projecting to the supraoptic nucleus express estrogen receptors. This provides immunocytochemical evidence that estrogen may regulate the activity of magnocellular oxytocin and vasopressin neurons in an indirect, trans-synaptic manner by influencing the activity of first-order neurons projecting to the supraoptic nucleus. The predominance of estrogen-receptive lamina terminalis and preoptic area inputs to the supraoptic nucleus suggests respective sites of estrogen action on magnocellular neurons in modulating fluid balance and reproductive function.