Nitric oxide via cGMP-dependent mechanisms increases dye coupling and excitability of rat supraoptic nucleus neurons

Fundamental Neurochemistry Review: The role of enteroendocrine cells in visceral pain

While visceral pain is commonly associated with disorders of the gut-brain axis, underlying mechanisms are not fully understood. Dorsal root ganglion (DRG) neurons innervate visceral structures and undergo hypersensitization in inflammatory models. The characterization of peripheral DRG neuron terminals is an active area of research, but recent work suggests that they communicate with enteroendocrine cells (EECs) in the gut. EECs sense stimuli in the intestinal lumen and communicate information to the brain through hormonal and electrical signaling. In that context, EECs are a target for developing therapeutics to treat visceral pain. Linaclotide is an FDA-approved treatment for chronic constipation that activates the intestinal membrane receptor guanylyl cyclase C (GUCY2C). Clinical trials revealed that linaclotide relieves both constipation and visceral pain. We recently demonstrated that the analgesic effect of linaclotide reflects the overexpression of GUCY2C on neuropod cells, a specialized subtype of EECs. While this brings some clarity to the relationship between linaclotide and visceral analgesia, questions remain about the intracellular signaling mechanisms and neurotransmitters mediating this communication. In this Fundamental Neurochemistry Review, we discuss what is currently known about visceral nociceptors, enteroendocrine cells, and the gut-brain axis, and ongoing areas of research regarding that axis and visceral pain.

Response to coincident inputs in electrically coupled primary afferents is heterogeneous and is enhanced by H-current (IH) modulation

Electrical synapses represent a widespread modality of interneuronal communication in the mammalian brain. These contacts, by lowering the effectiveness of random or temporally uncorrelated inputs, endow circuits of coupled neurons with the ability to selectively respond to simultaneous depolarizations. This mechanism may support coincidence detection, a property involved in sensory perception, organization of motor outputs, and improvement signal-to-noise ratio. While the role of electrical coupling is well established, little is known about the contribution of the cellular excitability and its modulations to the susceptibility of groups of neurons to coincident inputs. Here, we obtained dual whole cell patch-clamp recordings of pairs of mesencephalic trigeminal (MesV) neurons in brainstem slices from rats to evaluate coincidence detection and its determinants. MesV neurons are primary afferents involved in the organization of orofacial behaviors whose cell bodies are electrically coupled mainly in pairs through soma-somatic gap junctions. We found that coincidence detection is highly heterogeneous across the population of coupled neurons. Furthermore, combined electrophysiological and modeling approaches reveal that this heterogeneity arises from the diversity of MesV neuron intrinsic excitability. Consistently, increasing these cells' excitability by upregulating the hyperpolarization-activated cationic current (I_H) triggered by cGMP results in a dramatic enhancement of the susceptibility of coupled neurons to coincident inputs. In conclusion, the ability of coupled neurons to detect coincident inputs is critically shaped by their intrinsic electrophysiological properties, emphasizing the relevance of neuronal excitability for the many functional operations supported by electrical transmission in mammals. NEW & NOTEWORTHY We show that the susceptibility of pairs of coupled mesencephalic trigeminal (MesV) neurons to coincident inputs is highly heterogenous and depends on the interaction between electrical coupling and neuronal excitability. Additionally, upregulating the hyperpolarization-activated cationic current (I_H) by cGMP results in a dramatic increase of this susceptibility. The I_H and electrical synapses have been shown to coexist in many neuronal populations, suggesting that modulation of this conductance could represent a common strategy to regulate circuit operation supported by electrical coupling.

Nitric oxide as a messenger between neurons and satellite glial cells in dorsal root ganglia

Abnormal neuronal activity in sensory ganglia contributes to chronic pain. There is evidence that signals can spread between cells in these ganglia, which may contribute to this activity. Satellite glial cells (SGCs) in sensory ganglia undergo activation following peripheral injury and participate in cellular communication via gap junctions and chemical signaling. Nitric oxide (NO) is released from neurons in dorsal root ganglia (DRG) and induces cyclic GMP (cGMP) production in SCGs, but its role in SGC activation and neuronal excitability has not been explored. It was previously reported that induction of intestinal inflammation with dinitrobenzoate sulfonate (DNBS) increased gap junctional communications among SGCs, which contributed to neuronal excitability and pain. Here we show that DNBS induced SGC activation in mouse DRG, as assayed by glial fibrillary acidic protein upregulation. DNBS also upregulated cGMP level in SGCs, consistent with NO production. In vitro studies on intact ganglia from DNBS-treated mice showed that blocking NO synthesis inhibited both SGCs activation and cGMP upregulation, indicating an ongoing NO production. Application of NO donor in vitro induced SGC activation, augmented gap junctional communications, and raised neuronal excitability, as assessed by electrical recordings. The cGMP analog 8-Br-cGMP mimicked these actions, confirming the role of the NO-cGMP pathway in intraganglionic communications. NO also augmented Ca²⁺ waves propagation in DRG cultures. It is proposed that NO synthesis in DRG neurons increases after peripheral inflammation and that NO induces SGC activation, which in turn contributes to neuronal hyperexcitability. Thus, NO plays a major role in neuron-SGC communication.

Gaseous modulators in the control of the hypothalamic neurohypophyseal system

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are gaseous molecules produced by the brain. Within the hypothalamus, gaseous molecules have been highlighted as autocrine and paracrine factors regulating endocrine function. Therefore, in the present review, we briefly discuss the main findings linking NO, CO, and H2S to the control of body fluid homeostasis at the hypothalamic level, with particular emphasis on the regulation of neurohypophyseal system output.

Retrograde response in axotomized motoneurons: nitric oxide as a key player in triggering reversion toward a dedifferentiated phenotype

The adult brain retains a considerable capacity to functionally reorganize its circuits, which mainly relies on the prevalence of three basic processes that confer plastic potential: synaptic plasticity, plastic changes in intrinsic excitability and, in certain central nervous system (CNS) regions, also neurogenesis. Experimental models of peripheral nerve injury have provided a useful paradigm for studying injury-induced mechanisms of central plasticity. In particular, axotomy of somatic motoneurons triggers a robust retrograde reaction in the CNS, characterized by the expression of plastic changes affecting motoneurons, their synaptic inputs and surrounding glia. Axotomized motoneurons undergo a reprograming of their gene expression and biosynthetic machineries which produce cell components required for axonal regrowth and lead them to resume a functionally dedifferentiated phenotype characterized by the removal of afferent synaptic contacts, atrophy of dendritic arbors and an enhanced somato-dendritic excitability. Although experimental research has provided valuable clues to unravel many basic aspects of this central response, we are still lacking detailed information on the cellular/molecular mechanisms underlying its expression. It becomes clear, however, that the state-switch must be orchestrated by motoneuron-derived signals produced under the direction of the re-activated growth program. Our group has identified the highly reactive gas nitric oxide (NO) as one of these signals, by providing robust evidence for its key role to induce synapse elimination and increases in intrinsic excitability following motor axon damage. We have elucidated operational principles of the NO-triggered downstream transduction pathways mediating each of these changes. Our findings further demonstrate that de novo NO synthesis is not only "necessary" but also "sufficient" to promote the expression of at least some of the features that reflect reversion toward a dedifferentiated state in axotomized adult motoneurons.

Effects of nitric oxide on magnocellular neurons of the supraoptic nucleus involve multiple mechanisms

Physiological evidence indicates that the supraoptic nucleus (SON) is an important region for integrating information related to homeostasis of body fluids. Located bilaterally to the optic chiasm, this nucleus is composed of magnocellular neurosecretory cells (MNCs) responsible for the synthesis and release of vasopressin and oxytocin to the neurohypophysis. At the cellular level, the control of vasopressin and oxytocin release is directly linked to the firing frequency of MNCs. In general, we can say that the excitability of these cells can be controlled via two distinct mechanisms: 1) the intrinsic membrane properties of the MNCs themselves and 2) synaptic input from circumventricular organs that contain osmosensitive neurons. It has also been demonstrated that MNCs are sensitive to osmotic stimuli in the physiological range. Therefore, the study of their intrinsic membrane properties became imperative to explain the osmosensitivity of MNCs. In addition to this, the discovery that several neurotransmitters and neuropeptides can modulate their electrical activity greatly increased our knowledge about the role played by the MNCs in fluid homeostasis. In particular, nitric oxide (NO) may be an important player in fluid balance homeostasis, because it has been demonstrated that the enzyme responsible for its production has an increased activity following a hypertonic stimulation of the system. At the cellular level, NO has been shown to change the electrical excitability of MNCs. Therefore, in this review, we focus on some important points concerning nitrergic modulation of the neuroendocrine system, particularly the effects of NO on the SON.

Whole transcriptome organisation in the dehydrated supraoptic nucleus

The supraoptic nucleus (SON) is part of the central osmotic circuitry that synthesises the hormone vasopressin (Avp) and transports it to terminals in the posterior lobe of the pituitary. Following osmotic stress such as dehydration, this tissue undergoes morphological, electrical and transcriptional changes to facilitate the appropriate regulation and release of Avp into the circulation where it conserves water at the level of the kidney. Here, the organisation of the whole transcriptome following dehydration is modelled to fit Zipf's law, a natural power law that holds true for all natural languages, that states if the frequency of word usage is plotted against its rank, then the log linear regression of this is -1. We have applied this model to our previously published euhydrated and dehydrated SON data to observe this trend and how it changes following dehydration. In accordance with other studies, our whole transcriptome data fit well with this model in the euhydrated SON microarrays, but interestingly, fit better in the dehydrated arrays. This trend was observed in a subset of differentially regulated genes and also following network reconstruction using a third-party database that mines public data. We make use of language as a metaphor that helps us philosophise about the role of the whole transcriptome in providing a suitable environment for the delivery of Avp following a survival threat like dehydration.

Selective up-regulation of JunD transcript and protein expression in vasopressinergic supraoptic nucleus neurones in water-deprived rats

The magnocellular neurones (MCN) of the supraoptic nucleus (SON) undergo reversible changes during dehydration. We hypothesise that alterations in steady-state transcript levels might be partially responsible for this plasticity. In turn, regulation of transcript abundance might be mediated by transcription factors. We have previously used microarrays to identify changes in the expression of mRNAs encoding transcription factors in response to water deprivation. We observed down-regulation of 11 and up-regulation of 31 transcription factor transcripts, including members of the activator protein-1 gene family, namely c-fos, c-jun, fosl1 and junD. Because JunD expression and regulation within the SON has not been previously described, we have used in situ hybridisation and the quantitative reverse transcriptase-polymerase chain reaction to confirm the array results, demonstrating a significant increase in JunD mRNA levels following 24 and 72 h of water deprivation. Western blot and immunohistochemistry revealed a significant increase in JunD protein expression following dehydration. Double-staining fluorescence immunohistochemistry with a neurone-specific marker (NeuN) demonstrated that JunD staining is predominantly neuronal. Additionally, JunD immunoreactivity is observed primarily in vasopressin-containing neurones with markedly less staining seen in oxytocin-containing MCNs. Furthermore, JunD is highly co-expressed with c-Fos in MCNs of the SON following dehydration. These results suggest that JunD plays a role in the regulation of gene expression within MCNs of the SON in association with other Fos and Jun family members.

A soluble guanylate cyclase mediates negative signaling by ammonium on expression of nitrate reductase in Chlamydomonas

Nitrate assimilation in plants and related organisms is a highly regulated and conserved pathway in which the enzyme nitrate reductase (NR) occupies a central position. Although some progress has been made in understanding the regulation of the protein, transcriptional regulation of the NR gene (NIA1) is poorly understood. This work describes a mechanism for the ammonium-mediated repression of NIA1. We report the characterization of a mutant defective in the repression of NIA1 and NR in response to ammonium and show that a gene (CYG56) coding for a nitric oxide (NO)-dependent guanylate cyclase (GC) was interrupted in this mutant. NO donors, cGMP analogs, a phosphodiesterase inhibitor isobutylmethylxanthine (IBMX), and a calcium ionophore (A23187) repress the expression of NIA1 in Chlamydomonas reinhardtii wild-type cells and also repress the expression of other ammonium-sensitive genes. In addition, the GC inhibitors LY83,583 (6-anilino-5,8-quinolinedione) and ODQ (1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one) release cells from ammonium repression. Intracellular NO and cGMP levels were increased in the presence of ammonium in wild-type cells. In the cyg56 mutant, NIA1 transcription was less sensitive to NO donors and A23187, but responded like the wild type to IBMX. Results presented here suggest that CYG56 participates in ammonium-mediated NIA1 repression through a pathway that involves NO, cGMP, and calcium and that similar mechanisms might be occurring in plants.

The nitric oxide/cyclic guanosine monophosphate pathway modulates the inspiratory-related activity of hypoglossal motoneurons in the adult rat

Motoneurons integrate interneuronal activity into commands for skeletal muscle contraction and relaxation to perform motor actions. Hypoglossal motoneurons (HMNs) are involved in essential motor functions such as breathing, mastication, swallowing and phonation. We have investigated the role of the gaseous molecule nitric oxide (NO) in the regulation of the inspiratory-related activity of HMNs in order to further understand how neural activity is transformed into motor activity. In adult rats, we observed nitrergic fibers and bouton-like structures in close proximity to motoneurons, which normally lack the molecular machinery to synthesize NO. In addition, immunohistochemistry studies demonstrated that perfusion of animals with a NO donor resulted in an increase in the levels of cyclic guanosine monophosphate (cGMP) in motoneurons, which express the soluble guanylyl cyclase (sGC) in the hypoglossal nucleus. Modulators of the NO/cGMP pathway were micro-iontophoretically applied while performing single-unit extracellular recordings in the adult decerebrated rat. Application of a NO synthase inhibitor or a sGC inhibitor induced a statistically significant reduction in the inspiratory-related activity of HMNs. However, excitatory effects were observed by ejection of a NO donor or a cell-permeable analogue of cGMP. In slice preparations, application to the bath of a NO donor evoked membrane depolarization and a decrease in rheobase, which were prevented by co-addition to the bath of a sGC inhibitor. These effects were not prevented by reduction of the spontaneous synaptic activity. We conclude that NO from afferent fibers anterogradely modulates the inspiratory-related activity of HMNs by a cGMP-dependent mechanism in physiological conditions.

Activation of neuronal nitric oxide release inhibits spontaneous firing in adult gonadotropin-releasing hormone neurons: a possible local synchronizing signal

The activation of nitric oxide (NO) signaling pathways in hypothalamic neurons plays a key role in the control of GnRH secretion that is central to reproductive function. It is unknown whether NO directly modulates the firing behavior of GnRH neurons in the preoptic region of the mature brain. Using patch-clamp recordings from GnRH neurons expressing green fluorescent protein in adult mice brain slices, we demonstrate that the NO precursor, L-arginine (Arg), or the NO donor, diethylamine/NO, induced a robust and reversible reduction in the spontaneous firing activity of GnRH neurons, including bursting activity. The effects of L-Arg were prevented by the NO synthase inhibitor N omega-nitro-L-Arg methyl ester hydrochloride. Histochemical studies revealing a close anatomical relationship between neurons producing NO and GnRH perikarya, together with the loss of the L-Arg-mediated inhibition of GnRH neuronal activity via the selective blockade of neuronal NO synthase, suggested that the primary source of local NO production in the mouse preoptic region was neuronal. Synaptic transmission uncoupling did not alter the effect of NO, suggesting that NO affects the firing pattern of GnRH neurons by acting at a postsynaptic site. We also show that the NO-mediated changes in membrane properties in the GnRH neurons require soluble guanylyl cyclase activity and may involve potassium conductance. By revealing that NO is a direct modulator of GnRH neuronal activity, our results introduce the intriguing possibility that this gaseous neurotransmitter may be used by the sexual brain to modulate burst firing patterns. It may set into phase the bursting activity of GnRH neurons at key stages of reproductive physiology.

14-3-3 proteins within the hypothalamic-neurohypophyseal system of the osmotically stressed rat: transcriptomic and proteomic studies

The hypothalamic-neurohypophyseal system (HNS) mediates neuroendocrine responses to dehydration through the actions of the antidiuretic hormone vasopressin (VP) and the natriuetic peptide oxytocin (OT). VP and OT are synthesised as separate prepropeptide precursors in the cell bodies of magnocellular neurones in the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus, the axons of which innervate the posterior pituitary gland (PP). Dehydration evokes a massive release of both peptides into the circulation, and this is accompanied by a function-related remodelling of the HNS. Microarray studies on mRNAs differentially expressed in the SON revealed that transcripts encoding the Ywhag and Ywhaz isoforms of the 14-3-3 family of regulatory proteins, are increased in the rat SON by 3 days of water deprivation; findings that we have confirmed by the real-time polymerase chain reaction. Because there is no necessary proportionality between transcript and protein abundance, we next examined Ywhag and Ywhaz translation products throughout the HNS in parallel with 14-3-3 post-translational modification, which is known to be an important determinant of functional activity. Both proteins are robustly expressed in the SON in VP- and OT-containing neurones, but the abundance of neither changes with dehydration. However, the total level of Ywhaz protein is increased in the neurointermediate lobe of the pituitary (NIL, which includes the PP), in parallel with a basic post-translationally modified isoform, suggesting transport from the cell bodies of the SON of newly-synthesised protein and changes in its activity. The level of an acidic, probably phosphorylated, Ywhag isoform is down-regulated in the SON by dehydration, although total levels are unchanged. Finally, based on the presence of a phosphorylated 14-3-3 binding motif, we have identified a 14-3-3 binding partner, proteasome subunit, beta type 7, in the NIL. Thus, we suggest that, through complex transcriptional, and post-translational processes, 14-3-3 proteins are involved in the regulation or mediation of HNS plasticity following dehydration.

Evidence for nitric oxide role in the caudal neurosecretory system of the European flounder, Platichthys flesus

A neuromodulatory role for nitric oxide has been reported for magnocellular neuroendocrine cells in mammalian hypothalamus. We examined its potential as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the euryhaline flounder. Immunocytochemistry using an antibody raised against human neuronal nitric oxide synthase (NOS) indicated the presence of NOS in the Dahlgren cells. Quantitative RT-PCR, using a flounder-specific probe, revealed NOS mRNA expression in the CNSS. In July, though not in September, NOS mRNA expression was significantly higher in fish fully adapted to seawater, compared to freshwater-adapted fish. Following acute transfer of fish from freshwater to seawater, NOS mRNA expression was elevated at 8h and then recovered by 24h. In pharmacological experiments in vitro, application of NO donors (SNAP, SNP) caused an increase in electrical activity (firing frequency) of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The NOS substrate, l-arginine, led to increased firing frequency, cell recruitment and enhanced bursting activity. However, this effect was not blocked by the NOS inhibitor L-NAME. These findings suggest that NO acts as a modulator within the CNSS, potentially enhancing electrical activity and hence secretory output. A role in supporting adaptation to hyperosmotic conditions is also indicated.

Dehydration-induced proteome changes in the rat hypothalamo-neurohypophyseal system

The hypothalamo-neurohypophyseal system (HNS) mediates neuroendocrine responses to dehydration through the action of the antidiuretic hormone vasopressin (VP). VP is synthesized as part of a prepropeptide in magnocellular neurons of the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus. This precursor is processed during transport to axon terminals in the posterior pituitary gland, in which biologically active VP is stored until mobilized for secretion by electrical activity evoked by osmotic cues. During release, VP travels through the blood stream to specific receptor targets located in the kidney in which it increases the permeability of the collecting ducts to water, reducing the renal excretion of water, thus promoting water conservation. The HNS undergoes a dramatic function-related plasticity during dehydration. We hypothesize that alterations in steady-state protein levels might be partially responsible for this remodeling. We investigated dehydration-induced changes in the SON and pituitary neurointermediate lobe (NIL) proteomes using two-dimensional fluorescence difference gel electrophoresis. Seventy proteins were altered by dehydration, including 45 in the NIL and 25 in the SON. Using matrix-assisted laser desorption/ionization mass spectrometry, we identified six proteins in the NIL (four down, two up) and nine proteins in the SON (four up, five down) that are regulated as a consequence of chronic dehydration. Results for five of these proteins, namely Hsp1alpha (heat shock protein 1alpha), NAP22 (neuronal axonal membrane protein 22), GRP58 (58 kDa glucose regulated protein), calretinin, and ProSAAS (proprotein convertase subtilisin/kexin type 1 inhibitor), have been confirmed using independent methods such as semiquantitative Western blotting, two-dimensional Western blotting, enzyme-linked immunoassay, and immunohistochemistry. These proteins may have roles in regulating and effecting HNS remodeling.

A comprehensive description of the transcriptome of the hypothalamoneurohypophyseal system in euhydrated and dehydrated rats

The hypothalamoneurohypophyseal system (HNS) consists of the large peptidergic magnocellular neurons of the supraoptic hypo thalamic nucleus (SON) and the paraventricular hypothalamic nucleus (PVN), the axons of which course through the internal zone of the median eminence and terminate at blood capillaries of the posterior lobe of the pituitary gland. The HNS is a specialized brain neurosecretory apparatus responsible for the production of the antidiuretic peptide hormone vasopressin (VP). VP maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of VP from magnocellular neuron axon terminals in the posterior pituitary, which is accompanied by a plethora of changes in the morphology, electrophysiological properties, and biosynthetic and secretory activity of the HNS. We wish to understand this functional plasticity in terms of the differential expression of genes. We have therefore used microarrays to comprehensively catalog the genes expressed in the PVN, the SON and the neurointermediate lobe of the pituitary gland of control and dehydrated rats. Comparison of these gene lists has enabled us to identify transcripts that are regulated as a consequence of dehydration as well as RNAs that are enriched in the PVN or the SON. We suggest that these differentially expressed genes represent candidate regulators and effectors of HNS activity and remodeling.

Microarray screening of suppression subtractive hybridization-PCR cDNA libraries identifies novel RNAs regulated by dehydration in the rat supraoptic nucleus

The magnocellular neurons (MCNs) of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus are the principal site of biosynthesis of prepropeptide precursor of the antidiuretic hormone vasopressin (VP). This precursor is processed during anterograde axonal transportation to terminals in the posterior pituitary gland, where biologically active VP is stored until release into the general circulation in response to physiological activation of the SON by osmotic cues. By binding to V2-type receptors located in the kidney, VP decreases the amount of water lost in urine. Osmotic activation of the SON is accompanied by a dramatic morphological and functional remodeling. We have sought to understand the mechanistic basis of this plasticity in terms of the differential expression of genes. To identify such genes, we adopted an unbiased global approach based on suppressive subtractive hybridization-polymerase chain reaction (SSH-PCR) Using this method, we generated libraries of clones putatively differentially expressed in control vs. dehydrated SON. To rapidly screen these libraries, 1,152 clones were subjected to microarray analysis, resulting in the identification of 459 differentially expressed transcripts. cDNA clones corresponding to 56 of these RNAs were sequenced, revealing many of them to be novel expressed sequence tags (ESTs). Four transcripts were shown by in situ hybridization (ISH) to be significantly up- or downregulated in the SON after dehydration. These genes may represent novel effectors or mediators of SON physiological remodeling.

Developmental changes of nitric oxide synthase expression in the rat hypothalamoneurohypophyseal system

The present study investigated the immunohistochemical localization of neuronal nitric oxide synthase (nNOS) in the hypothalamoneurohypophyseal system (HNS) of the developing rats on postnatal day 1 (PN1), 7 (PN7), 14 (PN14), 21 (PN21), and the adult rats. The nNOS-positive neurons were not discernable in the supraoptic nucleus (SON), the paraventricular nucleus (PVN), and the median eminence (ME) at PN1 and PN7. A few neurons positive for nNOS were first detected at PN14. At PN21, the nNOS-positive cells in SON and PVN rapidly increased in number. The pattern of nNOS expression at this stage approached that of the adult. Moreover, the increase of nNOS expression in the SON and PVN during the postnatal period was accompanied by the maturation of arginine vasopressin (AVP) and oxytocin (OT) neurons as indicated by the number and size of OT or AVP neurons in the SON and PVN. The patterns of AVP versus OT expression also reached that of the adult by the end of the third postnatal week. The time course of the change in nNOS expression coincided with the maturation of AVP and OT neurons in the HNS and suggested that NO synthesized by conversion of NOS is involved in the modulation of activity of neurons in the SON and PVN of the HNS.

Update on connexins and gap junctions in neurons and glia in the mammalian nervous system

Among the 20 proposed members of the connexin family of proteins that form gap junctional intercellular communication (GJIC) channels in mammalian tissues, over half are reported to be expressed in the nervous system. There have been conflicting observations, however, concerning the particular connexins expressed by astrocytes, oligodendrocytes, Schwann cells and neurons. Identification of the several connexin proteins at gap junctions between each neuronal and glial cell type is essential for the rational design of investigations into the functions of GJIC between glial cells and into the functional contributions of electrical and "mixed" (chemical plus electrical) synapses to communication between neurons in the mammalian nervous system. In this report, we provide a summary of recent findings regarding the localization of connexins in gap junctions between glial cells and between neurons. Attention is drawn to technical considerations involved in connexin localization by light and electron microscope immunohistochemistry and to limitations of physiological methods and approaches currently used to analyze neuronal and glial coupling. Early physiological studies that provided evidence for the presence of gap junctions and electrical synapses in isolated regions of the mammalian brain and spinal cord are reexamined in light of recent evidence for widely expressed neuron-specific connexins and for the existence of several newly discovered types of gap junctions linking neurons.

Cellular sources, targets and actions of constitutive nitric oxide in the magnocellular neurosecretory system of the rat

Nitric oxide (NO) is a key activity-dependent modulator of the magnocellular neurosecretory system (MNS) during conditions of high hormonal demand. In addition, recent studies support the presence of a functional constitutive NO tone. The aim of this study was to identify the cellular sources, targets, signalling mechanisms and functional relevance of constitutive NO production within the supraoptic nucleus (SON). Direct visualization of intracellular NO, along with neuronal nitric oxide synthase (nNOS) and cGMP immunohistochemistry, was used to study the cellular sources and targets of NO within the SON, respectively. Our results support the presence of a strong NO basal tone within the SON, and indicate that vasopressin (VP) neurones constitute the major neuronal source and target of basal NO. NO induced-fluorescence and cGMP immunoreactivity (cGMPir) were also found in the glia and microvasculature of the SON, suggesting that they contribute as sources/targets of NO within the SON. cGMPir was also found in association with glutamic acid decarboxylase 67 (GAD67)- and vesicular glutamate transporter 2 (VGLUT2)-positive terminals. Glutamate, acting on NMDA and possibly AMPA receptors, was found to be an important neurotransmitter driving basal NO production within the SON. Finally, electrophysiological recordings obtained from SON neurones in a slice preparation indicated that constitutive NO efficiently restrains ongoing firing activity of these neurones. Furthermore, phasically active (putative VP) and continuously firing neurones appeared to be influenced by NO originating from different sources. The potential roles for basal NO as an autocrine signalling molecule, and one that bridges neuronal-glial-vascular interactions within the MNS are discussed.

Distribution of soluble guanylyl cyclase in the rat brain

The diffusible messenger nitric oxide (NO) acts in the brain largely through activation of soluble guanylyl cyclase (sGC), a heterodimer comprising alpha and beta subunits. We used immunohistochemistry to study the distribution of both sGC subunits in the brain of adult rats. alpha and beta subunits gave similar widespread staining throughout the CNS, which was strongest in neostriatum, olfactory tubercle, and supraoptic nucleus. Double-labeling experiments showed striking cellular colocalization in most brain regions, suggesting that the two subunits may be organized into enzymatically active alpha/beta heteromers. Mismatches were observed in cerebellar cortex: Purkinje cells and Bergmann glia were positive for both subunits, whereas granule cells and interneurons in the molecular layer were strongly immunopositive for beta but only weakly stained for the alpha subunit. By using multiple labeling, we compared the localization of sGC with neuronal nitric oxide synthase (NOS-I, the NO-producing enzyme in neurons). In forebrain, the distribution of sGC and NOS-I was complementary, with only occasional colocalization. In contrast, cellular colocalization was common in midbrain and cerebellum. These data support a widespread role for the NO/sGC/cGMP pathway in the CNS and suggest that, in addition to its role as paracrine messenger, NO may also be an intracellular autocrine agent.

Signalling pathway of nitric oxide in synaptic GABA release in the rat paraventricular nucleus

In the paraventricular nucleus (PVN) of the hypothalamus, nitric oxide (NO) inhibits sympathetic outflow through increased GABA release. However, the signal transduction pathways involved in its action remain unclear. In the present study, we determined the role of cGMP, soluble guanylyl cyclase, and protein kinase G in the potentiating effect of NO on synaptic GABA release to spinally projecting PVN neurones. The PVN neurones were retrogradely labelled by a fluorescent tracer injected into the thoracic spinal cord of rats. Whole-cell voltage-clamp recordings were performed on labelled PVN neurones in the hypothalamic slice. Bath application of the NO donor, S-nitroso-N-acetyl-penicillamine (SNAP), reproducibly increased the frequency of miniature GABAergic inhibitory postsynaptic currents (mIPSCs) without changing the amplitude and the decay time constant. Neither replacement of Ca2+ with Co2+ nor application of Cd2+ to block the Ca2+ channel altered the effect of SNAP on mIPSCs. Also, the effect of SNAP on mIPSCs was not significantly affected by thapsigargin, a Ca2+-ATPase inhibitor that depletes intracellular Ca2+ stores. Application of a membrane-permeant cGMP analogue, pCPT-cGMP, mimicked the effect of SNAP on mIPSCs in the presence of a phosphodiesterase inhibitor, IBMX. Furthermore, both the soluble guanylyl cyclase inhibitor, ODQ, and the specific protein kinase G inhibitor, Rp pCPT cGMP, abolished the effect of SNAP on mIPSCs. Thus, these data provide substantial new information that NO potentiates GABAergic synaptic inputs to spinally projecting PVN neurones through a cGMP-protein kinase G pathway.

NO inhibition of the magnocellular neuroendocrine system in rats is independent of cGMP signaling pathway

Our objective was to test the hypothesis that the cGMP signal-transduction mechanism mediates nitric oxide's (NO) modulation of oxytocin (OT) and vasopressin (VP) secretion from the hypothalamo-neurohypophysial system. Three studies were conducted in adult male Sprague-Dawley rats: (1a) Euhydrated rats received an intracerebroventricular (icv) infusion (1 microl/min for 30 min) of artificial cerebrospinal fluid (aCSF), vehicle (2.6% dimethyl sulfoxide [DMSO]) or 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) (0.05 microg/microl), an inhibitor of soluble guanylyl cyclase (sGC). ODQ did not affect basal levels of plasma VP or OT; (1b) Rats dehydrated for 24 h received aCSF or 8-Br-cGMP (icv), a membrane-permeable analog of cGMP, and plasma hormones were measured 2 min later. 8-Br-cGMP did not significantly change VP or OT levels; (2) Rats ingested water or 2% NaCl for 4 days, and NO synthase (NOS) and sGC activities were measured in posterior pituitaries, the anatomical site of hormone secretion. Salt loading enhanced (P < 0.001) production of [(14)C]citrulline, the coproduct of NO synthesis, without altering cGMP; (3) One SON was microdialyzed with [(14)C]arginine and NOS and sGC activities were quantified in microdialysates during intravenous (iv) infusion of isotonic or hypertonic saline in awake and anesthetized rats. In awake rats, [(14)C]citrulline recovery, but not cGMP, increased (P < 0.05) during intravenous infusion of both isotonic and hypertonic solutions, and after insertion of microdialysis probe itself. In anesthetized rats, however, where basal NOS activity is low, intravenous infusion of hypertonic, but not isotonic solution, increased [(14)C]citrulline recovery without affecting cGMP. Thus, in the forebrain, neither NO produced basally nor during osmotic stimulation depends on cGMP to modulate plasma vasopressin and oxytocin secretion.

Nitric oxide modulation of the hypothalamo-neurohypophyseal system

Nitric oxide (NO), a free radical gas produced endogenously from the amino acid L-arginine by NO synthase (NOS), has important functions in modulating vasopressin and oxytocin secretion from the hypothalamo-neurohypophyseal system. NO production is stimulated during increased functional activity of magnocellular neurons, in parallel with plastic changes of the supraoptic nucleus (SON) and paraventricular nucleus. Electrophysiological data recorded from the SON of hypothalamic slices indicate that NO inhibits firing of phasic and non-phasic neurons, while L-NAME, an NOS inhibitor, increases their activity. Results from measurement of neurohypophyseal hormones are more variable. Overall, however, it appears that NO, tonically produced in the forebrain, inhibits vasopressin and oxytocin secretion during normovolemic, isosmotic conditions. During osmotic stimulation, dehydration, hypovolemia and hemorrhage, as well as high plasma levels of angiotensin II, NO inhibition of vasopressin neurons is removed, while that of oxytocin neurons is enhanced. This produces a preferential release of vasopressin over oxytocin important for correction of fluid imbalance. During late pregnancy and throughout lactation, fluid homeostasis is altered and expression of NOS in the SON is down- and up-regulated, respectively, in parallel with plastic changes of the magnocellular system. NO inhibition of magnocellular neurons involves GABA and prostaglandin synthesis and the signal-transduction mechanism is independent of the cGMP-pathway. Plasma hormone levels are unaffected by i.c.v. 1H-[1, 2, 4]oxadiazolo-[4,3-a]quinoxalin-1-one (a soluble guanylyl cyclase inhibitor) or 8-Br-cGMP administered to conscious rats. Moreover, cGMP does not increase in homogenates of the neural lobe and in microdialysates of the SON when NO synthesis is enhanced during osmotic stimulation. Among alternative signal-transduction pathways, nitrosylation of target proteins affecting activity of ion channels is considered.

Nitric oxide and homeostatic control: an intercellular signalling molecule contributing to autonomic and neuroendocrine integration?

Accumulated evidence indicates that nitric oxide (NO) plays a pivotal role in the central control of bodily homeostasis, including cardiovascular and fluid balance regulation. Two major neuronal substrates mediating NO actions in the control of homeostasis are the paraventricular nucleus (PVN) of the hypothalamus, considered a key center for the integration of neuroendocrine and autonomic functions, and the supraoptic nucleus (SON). In this work, a comprehensive review of NO modulatory actions within the SON/PVN, including NO actions on neuroendocrine and autonomic outputs, as well as the cellular mechanisms underlying these effects is provided. Furthermore, this review comprises recent progress from our laboratory that adds to our current understanding of the cellular sources, targets and mechanisms underlying NO actions within neuroendocrine and autonomic hypothalamic neuronal circuits. By combining in vitro patch clamp recordings, tract-tracing neuroanatomy, immunohistochemistry and live imaging techniques, we started to shed light into the cellular sources and signals driving NO production within the SON and PVN, as well as NO actions and mechanisms targeting discrete neuronal populations within these circuits. Based on this new information, we have expanded one of the current working models in the field, highlighting a key role for NO as a signaling molecule that facilitates crosstalk among various cell types and systems. We propose that this dynamic NO signaling mechanisms may constitute a neuroanatomical and functional substrate underlying the ability of the SON and PVN to coordinate complex neuroendocrine and autonomic output patterns.

Deciphering the mechanisms of homeostatic plasticity in the hypothalamo-neurohypophyseal system—genomic and gene transfer strategies

The hypothalamo-neurohypophyseal system (HNS) is the specialised brain neurosecretory apparatus responsible for the production of a peptide hormone, vasopressin, that maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of hormone from the HNS, and this is accompanied by a plethora of changes in morphology, electrical properties and biosynthetic and secretory activity, all of which are thought to facilitate hormone production and delivery, and hence the survival of the organism. We have adopted a functional genomic strategy to understand the activity dependent plasticity of the HNS in terms of the co-ordinated action of cellular and genetic networks. Firstly, using microarray gene-profiling technologies, we are elucidating which genes are expressed in the HNS, and how the pattern of expression changes following physiological challenge. The next step is to use transgenic rats to probe the functions of these genes in the context of the physiological integrity of the whole organism.

The role of nitric oxide in morphine dependence and withdrawal excitation of rat oxytocin neurons

Magnocellular oxytocin neurons develop morphine dependence after intracerebroventricular infusion for 5 days as revealed by their profound excitation following naloxone-induced withdrawal. Oxytocin neurons strongly express nitric oxide synthase (NOS) and nitric oxide (NO) inhibits their activity. This study investigated whether excitation of oxytocin neurons during morphine withdrawal involves reduced activity of NOS and NO. Neuron activity was measured in urethane-anaesthetized rats with blood sampling for oxytocin radioimmunoassay and extracellular single unit firing rate recording of supraoptic nucleus oxytocin neurons. To compare morphine-dependent and -naive rats oxytocin secretion was measured during stimulation by intravenous hypertonic saline infusion. Prior treatment with Nomega-nitro-l-arginine methyl ester, a NOS inhibitor, facilitated osmotically stimulated oxytocin secretion in both morphine-dependent and -naive rats. The facilitation was not different between these groups when corrected for the slower responses observed in morphine-dependent rats. Treatment of morphine-dependent rats with Nomega-nitro-l-arginine methyl ester also enhanced oxytocin secretion during naloxone-precipitated withdrawal. Oxytocin neurons excited by withdrawal were recorded during microdialysis application to the supraoptic nucleus of the NO donor sodium nitroprusside alone and in combination with the GABAA antagonist bicuculline. Sodium nitroprusside inhibited oxytocin neurons during naloxone-precipitated morphine withdrawal and, while bicuculline alone increased firing rate, it did not reduce the inhibition by sodium nitroprusside, in contrast with previous findings in naive rats. Together, these findings indicate that NO restraint of oxytocin secretion is not curtailed during morphine dependence and remains a potent inhibitor of withdrawal excitation despite reduced effectiveness on GABA innervation of the supraoptic nucleus. Hence there is no evidence that changes in NO regulation underlie excitation of oxytocin neurons during opiate withdrawal in morphine dependence.

The effects of nitric oxide on magnocellular neurons could involve multiple indirect cyclic GMP-dependent pathways

Nitric oxide (NO) is known to regulate the release of arginine-vasopressin (AVP) and oxytocin (OT) by the paraventricular nucleus (PVN) and the supraoptic nucleus (SON). The aim of the current study was to identify in these nuclei the NO-producing neurons and the NO-receptive cells in mice. The determination of NO-synthesizing neurons was performed by double immunohistochemistry for the neuronal form of NO synthase (NOS), and AVP or OT. Besides, we visualized the NO-receptive cells by detecting cyclic GMP (cGMP), the major second messenger for NO, by immunohistochemistry on hypothalamus slices. Neuronal NOS was exclusively colocalized with OT in the PVN and the SON, suggesting that NO is mainly synthesized by oxytocinergic neurons in mice. By contrast, cGMP was not observed in magnocellular neurons, but in GABA-, tyrosine hydroxylase- and glutamate-positive fibers, as well as in GFAP-stained cells. The cGMP-immunostaining was abolished by incubating brain slices with a NOS inhibitor (L-NAME). Consequently, we provide the first evidence that NO could regulate the release of AVP and OT indirectly by modulating the activity of the main afferents to magnocellular neurons rather than by acting directly on magnocellular neurons. Moreover, both the NADPH-diaphorase activity and the mean intensity of cGMP-immunofluorescence were increased in monoamine oxidase A knock-out mice (Tg8) compared to control mice (C3H) in both nuclei. This suggests that monoamines could enhance the production of NO, contributing by this way to the fine regulation of AVP and OT release and synthesis.

Connexin immunoreactivity in glial cells of the rat retina

The rat retina contains two types of macroglial cells, Müller cells, radial glial cells that are the principal macroglial cells of vertebrate retinas, and astrocytes associated with the surface vasculature. In addition to the often-described gap-junctional coupling between astrocytes, coupling also occurs between astrocytes and Müller cells. Immunohistochemistry and confocal microscopy were used to identify connexins in the retinas of pigmented rats. Several antibodies directed against connexin43 stained astrocytes, identified using antibodies directed against glial fibrillary acidic protein (GFAP). In addition, two connexin43 antibodies stained Müller cells, identified with antibodies directed against S100 or glutamine synthetase. Connexin30-immunoreactive puncta were confined to the vitreal surface of the retina and colocalized with GFAP-immunoreactive astrocyte processes. Connexin45 immunoreactivity was associated with both astrocytes and Müller cells. We conclude that retinal glial cells express multiple connexins, and the patterns of immunostaining that we observe in this study are consistent with the expression of connexins30, -43, and possibly -45 by astrocytes and the expression of connexins43 and -45 by Müller cells. As gap-junction channels may be formed by both homotypic and heterotypic hemichannels, and the hemichannels may themselves be homomeric or heteromeric, there exists a multitude of possible gap-junction channels that could underlie the homotypic coupling between retinal astrocytes and the heterotypic coupling between astrocytes and Müller cells.

Mesencephalic trigeminal neurons are innervated by nitric oxide synthase-containing fibers and respond to nitric oxide

In the present study we found that mesencephalic trigeminal (Mes-V) neurons of the rat are innervated by nitrergic fibers and that nitric oxide (NO) modifies the electrophysiological properties of these cells. Mes-V neurons were surrounded by a network of fibers that contained neuronal nitric oxide synthase (nNOS); these fibers gave rise to terminal-, bouton-like structures which ended in Mes-V cells bodies. These cells, which did not display nNOS-like immunoreactivity were immunoreactive to a cGMP antibody. By performing intracellular recordings in the adult rat brain slice preparation, the effects of diethylenetriamine/NO adduct (DETA/NO) applications were examined. DETA/NO induced a depolarization that averaged 2.2 mV (range: 1-6 mV) in nine of 22 neurons. In 15 of 22 neurons (68% of the cells), there was a decrease in current threshold from 0.74 to 0.60 nA (19%; P<0.001). The excitatory effects of DETA/NO were abolished by ODQ, a blocker of soluble guanylate cyclase. Input resistance (R(in)) decreased in 80% of the cells from a mean of 24.8 to 20.6 Momega (17%; P<0.001) and the membrane time constant (tau(m)) decreased from 7.5 to 5.6 ms (25%; P<0.05). The 'sag' seen in the membrane response of these cells to current pulses was augmented during DETA/NO application. These findings indicate that there is a nitrergic innervation of Mes-V neurons and that these sensory cells are target for NO that may act on them as an excitatory neuromodulator promoting the synthesis of intracellular cGMP.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.

Flufenamic acid blocks depolarizing afterpotentials and phasic firing in rat supraoptic neurones

Depolarizing afterpotentials (DAPs) that follow action potentials in magnocellular neurosecretory cells (MNCs) are thought to underlie the generation of phasic firing, a pattern that optimizes vasopressin release from the neurohypophysis. Previous work has suggested that the DAP may result from the Ca(2+)-dependent reduction of a resting K(+) conductance. Here we examined the effects of flufenamic acid (FFA), a blocker of Ca(2+)-dependent non-selective cation (CAN) channels, on DAPs and phasic firing using intracellular recordings from supraoptic MNCs in superfused explants of rat hypothalamus. Application of FFA, but not solvent (0.1 % DMSO), reversibly inhibited (IC(50) = 13.8 microM; R = 0.97) DAPs and phasic firing with a similar time course, but had no significant effects (P > 0.05) on membrane potential, spike threshold and input resistance, nor on the frequency and amplitude of spontaneous synaptic potentials. Moreover, FFA did not affect (P > 0.05) the amplitude, duration, undershoot, or frequency-dependent broadening of action potentials elicited during the spike trains used to evoke DAPs. These findings suggest that FFA inhibits the DAP by directly blocking the channels responsible for its production, rather than by interfering with Ca(2+) influx. They also support a role for DAPs in the generation of phasic firing in MNCs. Finally, the absence of a depolarization and increased membrane resistance upon application of FFA suggests that the DAP in MNCs may not be due to the inhibition of resting K(+) current, but to the activation of CAN channels.

Histamine H1-receptor modulation of inter-neuronal coupling among vasopressinergic neurons depends on nitric oxide synthase activation

Inter-neuronal coupling is a relatively recently documented property of a wide variety of cell groups in the mammalian central nervous system. For many of these groups there is evidence that the coupling can be modulated by synaptic inputs. Incidence of dye coupling among vasopressin (VP) neurons of the rat supraoptic nucleus (SON) has been shown to increase in response to either activation of histamine H(1)-receptors or to increased NO production. Both of these effects involve activation of cGMP-dependent pathways. We tested the hypothesis that activation of H(1)-receptors resulted in downstream activation of NO synthase, which then mediated the H(1)-receptor effects. Putative VP neurons were intracellularly recorded and dye-injected in horizontal slices of hypothalamus, in which monosynaptic connections from the tuberomammillary nucleus (TM) were intact and electrically stimulated. Single-pulse TM stimulation evoked EPSPs and repetitive stimulation resulted in a nearly 3-fold increase in coupling incidence over unstimulated slices. TM-stimulated increased coupling was completely blocked by inhibitors of NO synthase (L-NAME) or of soluble guanylyl cyclase (ODQ or methylene blue), or pyrilamine, suggesting that the H(1)-receptor is not directly linked to guanylyl cyclase. Addition of the NO precursor, L-arginine or the NO donor, SNP, in combination with TM stimulation produced increases in coupling that were not significantly larger than those seen with stimulation alone, supporting the idea that a common pathway was used. We conclude that H(1)-receptors activate NO synthase via G-protein-coupled pathways and that NO working though its receptor, induces the downstream cGMP-dependent processes that result in increased inter-neuronal coupling.

Peripartum interneuronal coupling in the supraoptic nucleus

In the supraoptic nucleus (SON), the incidence of conducting gap junctions (gjs), as indicated by dye coupling, is low in cycling females, but dramatically elevated in nursing mothers. Functionally, this is consistent with the well-established presence of synchronous milk ejection bursts among oxytocin neurons only in the lactating rat. In situ hybridization data, however, revealed elevated gj mRNA expression on the last day of pregnancy, a time when burst firing by putative oxytocin neurons is absent. Using Lucifer Yellow dye coupling, we determined the incidence of high conductance gjs in SONs of proestrous, immediately prepartum, postpartum non-lactating, lactating day 1, and lactating day 9-10 rats. Results indicate that coupling incidence is high only at times when milk ejection bursts are known to occur, and that the elevated gj mRNA expression seen on the last day of pregnancy does not indicate conducting gjs. It is suggested that gj conductance states, but not gj expression, are modulated by plasma estradiol titers.

Role of the supraoptic nucleus in regulation of parturition and milk ejection revisited

This review will focus on the activity of oxytocin neurons in the supraoptic nucleus (SON) and some factors that regulate their function during parturition and milk ejection in the rat. The level of oxytocin increases in the blood during parturition following a regression of the corpus luteum. The increase in oxytocin secretion is presumably a consequence of releasing the oxytocin neurons from restraining inhibitory influences of endogenous opioids-, nitric oxide-, and GABA-containing neurons following declining blood levels of progesterone on the one hand and increasing levels of estrogen on the other during late pregnancy. However, the principal stimulus for the increased oxytocin release is believed to originate, at least in part, from mechanical stimulation to the uterine cervix by fetuses near term, the resultant uterine contractile activity, and the fetal expulsion reflex. Hence, the contractile activity of the uterus acts through positive feedback mechanisms during parturition to stimulate oxytocin neurons as well, and this further increases the secretion of oxytocin. During suckling in lactating rats, somatosensory stimuli from the pups induce intermittent synchronized burst firing of oxytocin neurons, resulting in pulsatile increases in blood oxytocin concentrations to cause milk ejection. The oxytocin neurons appear to have an intrinsic capability to fire in a bursting fashion as determined by observation of this phenomenon in brain slice or tissue culture preparations. The release of oxytocin within the microenvironment of the SON and paraventricular nucleus coupled with morphological reorganization in these nuclei play important roles in the bursting activity of each oxytocin neuron and synchronization in vivo. However, the mechanism responsible for the synchronization of electrical activity in oxytocin neurons in the four discrete hypothalamic nuclei remains an interesting unanswered question.

Gene expression in the supraoptic nucleus

The magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) express multiple kinds of genes, including not only the classical hormones arginine vasopressin (AVP) and oxytocin (OXT), but also other physiologically active substances including neuropeptides, their receptors, and nitric oxide (NO) synthase, the rate-limiting enzyme in the synthesis of NO under physiological condition. For example, osmotic stimuli such as dehydration and chronic salt loading cause a wide range of changes of the expression levels of the genes and marked induction of the expression of the genes in the SON. The expression of the NO synthase gene in the SON under physiological conditions is reviewed.

Activity-related, dynamic neuron-glial interactions in the hypothalamo-neurohypophysial system

Magnocellular neurons located in the supraoptic nucleus send their principal axons to terminate in the neurohypophysis, where they release vasopressin and oxytocin into the blood circulation. This magnocellular hypothalamo-neurohypophysial system is known to undergo dramatic activity-dependent structural plasticity during chronic physiological stimulation, such as dehydration and lactation. This structural plasticity is accompanied not only by synaptic remodeling, increased direct neuronal membrane apposition, and dendritic bundling in the supraoptic nucleus, but also organization of neurovascular contacts in the neurohypophysis. The adjacent glial cells actively participate in these plastic changes in addition to magnocellular neurons themselves. Many molecules that are possibly concerned with dynamic structural remodeling are highly expressed in the hypothalamo-neurohypophysial system, although they are generally at low expression levels in other regions of adult brains. Interestingly, some of them are highly expressed only in embryonic brains. On the basis of function, these molecules are classified mainly into two categories. Cytoskeletal proteins, such as tubulin, microtubule-associated proteins, and intermediate filament proteins, are responsible for changing both glial and neuronal morphology and location. Cell adhesion molecules, belonging to immunoglobulin superfamily proteins and extracellular matrix glycoproteins, also participate in neuronal-glial, neuronal-neuronal, and glial-glial recognition and guidance. Thus, the hypothalamo-neurohypophysial system is an interesting model for elucidating physiological significance and molecular mechanisms of activity-dependent structural plasticity in adult brains.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

NO inhibits supraoptic oxytocin and vasopressin neurons via activation of GABAergic synaptic inputs

To study modulatory actions of nitric oxide (NO) on GABAergic synaptic activity in hypothalamic magnocellular neurons in the supraoptic nucleus (SON), in vitro and in vivo electrophysiological recordings were obtained from identified oxytocin and vasopressin neurons. Whole cell patch-clamp recordings were obtained in vitro from immunochemically identified oxytocin and vasopressin neurons. GABAergic synaptic activity was assessed in vitro by measuring GABA(A) miniature inhibitory postsynaptic currents (mIPSCs). The NO donor and precursor sodium nitroprusside (SNP) and L-arginine, respectively, increased the frequency and amplitude of GABA(A) mIPSCs in both cell types (P < or = 0.001). Retrodialysis of SNP (50 mM) onto the SON in vivo inhibited the activity of both neuronal types (P < or = 0.002), an effect that was reduced by retrodialysis of the GABA(A)-receptor antagonist bicuculline (2 mM, P < or = 0.001). Neurons activated by intravenous infusion of 2 M NaCl were still strongly inhibited by SNP. These results suggest that NO inhibition of neuronal excitability in oxytocin and vasopressin neurons involves pre- and postsynaptic potentiation of GABAergic synaptic activity in the SON.

Cellular communication and coupling within the suprachiasmatic nucleus

In mammals, the part of the nervous system responsible for most circadian behavior can be localized to a pair of structures in the hypothalamus known as the suprachiasmatic nucleus (SCN). Importantly, when SCN neurons are removed from the organism and maintained in a brain slice preparation, they continue to generate 24h rhythms in electrical activity, secretion, and gene expression. Previous studies suggest that the basic mechanism responsible for the generation of these rhythms is intrinsic to individual cells in the SCN. If we assume that individual cells in the SCN are competent circadian oscillators, it is obviously important to understand how these cells communicate and remain synchronized with each other. Cell-to-cell communication is clearly necessary for conveying inputs to and outputs from the SCN and may be involved in ensuring the high precision of the observed rhythm. In addition, there is a growing body of evidence that a number of systems-level phenomena could be dependent on the cellular communication between circadian pacemaker neurons. It is not yet known how this cellular synchronization occurs, but it is likely that more than one of the already proposed mechanisms is utilized. The purpose of this review is to summarize briefly the possible mechanisms by which the oscillatory cells in the SCN communicate with each other.

Nitric oxide as modulator of neuronal function

The gas NO is a messenger that modulates neuronal function. The use of NO donors and NO synthase inhibitors as pharmacological tools revealed that this free radical is probably implicated in the regulation of excitability and firing, in long-term potentiation and long-term depression, as well as in memory processes. Moreover, NO modulates neurotransmitter release. In vivo and in vitro studies have shown that, in all brain structures investigated, endogenous NO modulates the release of several neurotransmitters, such as acetylcholine, catecholamines, excitatory and inhibitory amino acids, serotonin, histamine, and adenosine. In most cases, enhanced NO level in the tissue increases the release of neurotransmitters, although decreasing effects have also been observed. Cyclic 3'-5' guanosine monophosphate and glutamate mediate the modulation of transmitter release by NO. Recent observations suggest that the release of some transmitters is dually influenced by NO. Thus, besides modulation by presynaptically located auto- and heteroreceptors, NO released from nitrergic neurons seems to play a universal role in modulating the release of transmitters in the brain.

Ionotropic histamine receptors and H2 receptors modulate supraoptic oxytocin neuronal excitability and dye coupling

Histaminergic neurons of the tuberomammillary nucleus (TM) project monosynaptically to the supraoptic nucleus (SON). This projection remains intact in our hypothalamic slices and permits investigation of both brief synaptic responses and the effects of repetitively activating this pathway. SON oxytocin (OX) neurons respond to single TM stimuli with fast IPSPs, whose kinetics resemble those of GABA(A) or glycine receptors. IPSPs were blocked by the Cl(-) channel blocker picrotoxin, but not by bicuculline or strychnine, and by histamine H(2), but not by H(1) or H(3) receptor antagonists, suggesting the presence of an ionotropic histamine receptor and the possible nonspecificity of currently used H(2) antagonists. G-protein mediation of the IPSPs was ruled out using guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS), pertussis toxin, and Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPs), none of which blocked evoked IPSPs. We also investigated the effects of synaptically released histamine on dye coupling and neuronal excitability. One hundred seventy-three OX neurons were Lucifer yellow-injected in horizontal slices. Repetitive TM stimulation (10 Hz, 5-10 min) reduced coupling, an effect blocked by H(2), but not by H(1) or H(3), receptor antagonists. Because H(2) receptors are linked to activation of adenylyl cyclase, TM-stimulated reduction in coupling was blocked by GDP-betaS, pertussis toxin, and Rp-cAMPs and was mimicked by 8-bromo-cAMP, 3-isobutyl-1-methylxanthine, and Sp-cAMP. Membrane potentials of OX and vasopressin neurons were hyperpolarized, accompanied by decreased conductances, in response to bath application of 8-bromo-cAMP but not the membrane-impermeable cAMP. These results suggest that synaptically released histamine, in addition to evoking fast IPSPs in OX cells, mediates a prolonged decrease in excitability and uncoupling of the neurons.

Nociceptive stimulation increases NO synthase mRNA and vasopressin heteronuclearRNA in the rat paraventricular nucleus

Nociceptive stimulation causes neuroendocrine responses such as arginine vasopressin (AVP) release and activation of the hypothalamo-pituitary-adrenal (HPA) axis. We examined the effects of nociceptive stimulation on the expression levels of neuronal nitric oxide synthase (nNOS) mRNA, heteronuclear (hn)RNA for AVP and AVP mRNA in the rat paraventricular nucleus (PVN) and supraoptic nucleus (SON), using in situ hybridization histochemistry. For nociceptive stimulation, formalin (5%) or saline was injected subcutaneously (s.c.) into the bilateral hind paws of rats. The expression of the nNOS gene in the PVN was significantly increased 2 and 6 h after s.c. injection of formalin in comparison with that in untreated and saline injected rats. The expression of the nNOS gene in the SON did not change in the untreated, saline- and formalin-injected rats. The AVP hnRNA in the PVN and SON was also significantly increased 15, 30 min and 2 h after s.c. injection of formalin, though AVP mRNA did not change at any time points that we studied. Plasma concentration of AVP was significantly increased 15 min after s.c. injection of formalin. These results suggest that NO in the PVN may be involved in nociceptive stimulation-induced neuroendocrine responses.

Confocal microscopic study of glial-vascular relationships in the retinas of pigmented rats

Astroglia are interposed between the cerebral vasculature and neurons, where they may mediate the transfer of substances from the circulation to neurons and couple changes in neuronal activity to changes in cerebral blood flow. The retina is a particularly advantageous model system for studying glial-vascular interactions in situ. Confocal microscopy and three-dimensional image reconstruction were used to study the anatomical relationships between glia and the surface vasculature in retinas acutely isolated from adult pigmented rats. Retinas were immunostained using antibodies directed against the basal lamina surrounding the vasculature as well as antibodies directed against glial fibrillary acidic protein. Surface vessels of all calibers were contacted by the processes of astrocytes. The vitreal surfaces of the large retinal vessels were covered by a meshwork of immunoreactive astrocyte processes of a variety of shapes, whereas the scleral surfaces of the vessels were supported by thick bundles of astrocyte processes. In addition, glial cells were filled intracellularly with the gap junction-permeable tracers Lucifer yellow and Neurobiotin. Intracellular fills clearly demonstrated the presence of astrocytes with somata that were closely apposed to the large retinal vessels. Tracer-filled astrocytes displayed a variety and complexity of shapes that was not apparent in immunostained material. Gap junctional coupling was stronger between astrocytes adjacent to the same artery than between periarterial astrocytes and astrocytes located away from arteries. Significantly fewer Müller cells were labeled when Neurobiotin was injected into astrocytes associated with arteries than when Neurobiotin was injected into astrocytes that were distant from arteries.

Dopamine-mediated regulation of striatal neuronal and network interactions

The dopaminergic system exerts dynamic modulation of glutamatergic afferent drive that is dependent on the temporal pattern of the dopaminergic input and the subtypes of striatal neurons affected. The differences in feedforward inhibition between striatal neurons comprising the direct and indirect output pathway confer distinct response-pattern differences in their respective targets,supporting brief bursts of activity in Type-I neurons but attenuating repetitive activity in Type-II cells. This temporal patterning is further modulated by NO-mediated signaling, and by tonic and phasic dopamine-mediated stimulation, which exerts preferential actions on indirect and direct output neurons, respectively. As a result,the striatal network is forced into state-dependent patterns of activity that differentially regulate muscle tone and voluntary motor activity via distinct output projections from the striatum.

Nitric oxide and the oxytocin system in pregnancy

We examined the functional role of the nitric oxide (NO)-producing system in magnocellular neurons and how this changes at the end of pregnancy, using a combination of blood sampling and oxytocin radioimmunoassay, electrophysiology, immunocytochemistry for Fos expression, and in situ hybridization histochemistry. In urethane-anesthetized virgin rats, systemic administration of NO synthase (NOS) inhibitors led to a facilitation of oxytocin release evoked by hyperosmotic stimulation. Direct application of the NO donor sodium nitroprusside to the supraoptic nucleus by in vivo microdialysis inhibited the electrical activity of both oxytocin neurons and vasopressin neurons, whereas direct application of an NOS inhibitor increased electrical activity, indicating that endogenous NO acts within the supraoptic nucleus to inhibit neuronal activity. However, during late pregnancy, the influence of endogenous NO is dramatically downregulated, reflected by a reduced expression of neuronal NOS mRNA in these neurons and a loss of efficacy of NOS inhibitors on stimulus-evoked oxytocin release. This downregulation may cause the oxytocin system to become more excitable at term, resulting in the capacity for greater release of oxytocin during parturition.

Expression of the IgLON cell adhesion molecules Kilon and OBCAM in hypothalamic magnocellular neurons

The vasopressin (AVP) and oxytocin (OXT) magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular nuclei (PVN) display reversible structural plasticity of neurons and glial cells under different conditions of neuropeptide secretion. In the present study, we investigated the expression of two immunoglobulin superfamily (IgSF) proteins, Kilon and OBCAM, in the magnocellular neurons by using monoclonal antibodies. Anti-Kilon antibody reacted specifically with the bacterially expressed recombinant Kilon but not with the recombinant OBCAM, and similarly anti-OBCAM antibody specifically recognized the recombinant OBCAM. Western blotting analysis revealed the specific expression of Kilon and OBCAM in the SON homogenates. Although Kilon and OBCAM of the SON homogenates were present as the insoluble form, most Kilon was present in the Triton-insoluble fraction, and OBCAM was localized mainly in the Triton-soluble fraction. Immunocytochemistry revealed Kilon and OBCAM immunoreactivity in the magnocellular neurons of the SON and PVN of the rat hypothalamus compared with outside of the SON and PVN in the hypothalamus. The double-labeling study with confocal microscopy further demonstrated that Kilon immunoreactivity was observed mainly in the dendrites of AVP-secreting neurons and also occasionally OXT-secreting neurons. However, OBCAM immunoreactivity was exclusively seen in the dendrites of AVP-secreting magnocellular neurons. Chronic physiological stimulation by 2% NaCl had no effect on the expression levels of either IgLON protein in the SON. Our study thus demonstrated specific expression of Kilon and OBCAM in the hypothalamic magnocellular neurons, particularly in dendrites, suggesting that they confer on magnocellular neurons the ability to rearrange dendritic connectivity.

Histamine suppresses non-NMDA excitatory synaptic currents in rat supraoptic nucleus neurons

Whole cell patch-clamp recordings were obtained from supraoptic neurons to investigate the effects of histamine on excitatory postsynaptic currents evoked by electrical stimulation of areas around the posterior supraoptic nucleus. When cells were voltage-clamped at -70 mV, evoked excitatory postsynaptic currents had amplitudes of 88.4 +/- 9.6 pA and durations of 41.1 +/- 3.0 ms (mean +/- SE; n = 43). With twin stimulus pulses (20 Hz) used, paired-pulse facilitation ratios were 1.93 +/- 0.12. Bath application of 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX) abolished synaptic currents. Histamine at concentrations approximately 0.1-10 microM reversibly suppressed excitatory postsynaptic currents in all supraoptic neurons tested. Within 2 min after application of (10 microM) histamine, current amplitudes and durations decreased by 61. 5 and 31.0%, respectively, with little change in the paired-pulse facilitation ratio. Dimaprit or imetit (H(2) or H(3) receptor agonists) did not reduce synaptic currents, whereas pyrilamine (H(1) receptor antagonist) blocked histamine-induced suppression of synaptic currents. When patch electrodes containing guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) were used to record cells, histamine still suppressed current amplitudes by 49.1% and durations by 41.9%. Similarly, intracellular diffusion of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) and H(7) did not abolish histamine-induced suppression of synaptic currents, either. Bath perifusion of 8-bromo-quanosine 3',5'-cyclic monophosphate reduced current amplitudes by 32.3% and durations by 27.9%. After bath perfusion of slices with N(omega)-nitro-L-arginine methyl ester (L-NAME), histamine injection decreased current amplitudes only by 31.9%, much less than the inhibition rate in control (P < 0.01). In addition, histamine induced little change in current durations and paired-pulse facilitation ratios, representing a partial blockade of histamine effects on synaptic currents by L-NAME. In supraoptic neurons recorded using electrodes containing BAPTA and perifused with L-NAME, the effects of histamine on synaptic currents were completely abolished. Norepinephrine injection reversibly decreased current amplitudes by 39.1% and duration by 64.5%, with a drop in the paired-pulse facilitation ratio of 47.9%. Bath perifusion of L-NAME, as well as intracellular diffusion of GDP-beta-S, 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, or BAPTA, failed to block norepinephrine-induced suppression of evoked synaptic currents. The present results suggest that histamine suppresses non-N-methyl-D-aspartate synaptic currents in supraoptic neurons through activation of H(1) receptors. It is possible that histamine first acts at supraoptic cells (perhaps both neuronal and nonneuronal) and induces the production of nitric oxide, which then diffuses to nearby neurons and modulates synaptic transmission by a postsynaptic mechanism.

Amphetamine withdrawal alters bistable states and cellular coupling in rat prefrontal cortex and nucleus accumbens neurons recorded in vivo

Repeated amphetamine administration is known to produce changes in corticoaccumbens function that persist beyond termination of drug administration. We have found previously that long-term alteration in dopamine systems leads to changes in gap junction communication, expressed as dye coupling, between striatal neurons. In this study, the cellular bases of amphetamine-induced changes were examined using in vivo intracellular recordings and dye injection in ventral prefrontal-accumbens system neurons of control and amphetamine-treated rats. Rats that had been withdrawn from repeated amphetamine displayed a significant increase in the incidence of dye coupling in the prefrontal cortex and nucleus accumbens, which persisted for up to 28 d after withdrawal. The increased coupling was limited to projection neurons in both prefrontal cortical and accumbens brain regions, as identified by their axonal trajectory or the absence of interneuron-selective immunocytochemical markers. These changes occurred with no substantial loss of tyrosine hydroxylase-immunoreactive terminals in these cortical and accumbens regions, ruling out dopamine degeneration as a precipitating factor. Previous studies showed that nitric oxide plays a role in the regulation of coupling; however, amphetamine-withdrawn rats had fewer numbers of neurons and processes that stained for nitric oxide synthase immunoreactivity. In amphetamine-treated rats, a higher proportion of cortical cells fired in bursts, and a larger proportion of accumbens and prefrontal cortical neurons exhibited bistable membrane oscillations. By increasing corticoaccumbens transmission, amphetamine withdrawal may lead to neuronal synchronization via gap junctions. Furthermore, this adaptation to amphetamine treatment persists long after the drug is withdrawn.